I've not found a way to do it directly, but some manipulation can be done by using system breaks. Note that spacers before and/or after will change the length of the multibar rest but NOT the actual printed symbol within it.
<wait a bit...>
Right, thinking about it, a layered staff containing only rests moved "off the page" and everything else hidden will work. N.B. the rests MUST be visible, but using Global_Mod they can be moved off the page so they don't print.
<wait a bit more...>
OK, just did a test: A layered staff containing only rests but with everything hidden, including the rests, can also be used, and with more control. On the hidden staff you add spacers in one of the bars that correspond with the multibar rest you want to modify. This can be used to fairly finely control the length of the multibar rest.
!NoteWorthyComposer(2.751)
|Editor|ActiveStaff:1|CaretIndex:1|CaretPos:0
|SongInfo|Title:""|Author:"<Name>"|Lyricist:""|Copyright1:"Copyright © 2022 <Name>"|Copyright2:"All Rights Reserved"
|PgSetup|StaffSize:16|Zoom:4|TitlePage:Y|JustifyVertically:Y|PrintSystemSepMark:N|ExtendLastSystem:N|DurationPadding:Y|PageNumbers:0|StaffLabels:None|BarNumbers:None|StartingBar:1
|Font|Style:StaffItalic|Typeface:"Times New Roman"|Size:10|Bold:Y|Italic:Y|CharSet:0
|Font|Style:StaffBold|Typeface:"Times New Roman"|Size:8|Bold:Y|Italic:N|CharSet:0
|Font|Style:StaffLyric|Typeface:"Times New Roman"|Size:7.2|Bold:N|Italic:N|CharSet:0
|Font|Style:PageTitleText|Typeface:"Times New Roman"|Size:24|Bold:Y|Italic:N|CharSet:0
|Font|Style:PageText|Typeface:"Times New Roman"|Size:12|Bold:N|Italic:N|CharSet:0
|Font|Style:PageSmallText|Typeface:"Times New Roman"|Size:8|Bold:N|Italic:N|CharSet:0
|Font|Style:User1|Typeface:"Times New Roman"|Size:8|Bold:N|Italic:N|CharSet:0
|Font|Style:User2|Typeface:"Times New Roman"|Size:8|Bold:N|Italic:N|CharSet:0
|Font|Style:User3|Typeface:"Times New Roman"|Size:8|Bold:N|Italic:N|CharSet:0
|Font|Style:User4|Typeface:"Times New Roman"|Size:8|Bold:N|Italic:N|CharSet:0
|Font|Style:User5|Typeface:"Times New Roman"|Size:8|Bold:N|Italic:N|CharSet:0
|Font|Style:User6|Typeface:"Times New Roman"|Size:8|Bold:N|Italic:N|CharSet:0
|PgMargins|Left:1.27|Top:1.27|Right:1.27|Bottom:1.27|Mirror:N
|AddStaff|Name:"Staff"|Group:"Standard"
|StaffProperties|EndingBar:Section Close|Visible:Y|BoundaryTop:12|BoundaryBottom:12|Lines:5|WithNextStaff:Layer|Color:Default
|StaffProperties|Muted:N|Volume:127|StereoPan:64|Device:0|Channel:1
|StaffInstrument|Trans:0|DynVel:10,30,45,60,75,92,108,127
|Clef|Type:Treble
|Key|Signature:C|Tonic:C
|TimeSig|Signature:4/4
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Bar
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Bar
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Bar
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Bar|SysBreak:Y
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Bar
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Bar
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Bar
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Bar|SysBreak:Y
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Bar
|RestMultiBar|NumBars:6|PrintOnce:N|WhenHidden:ShowBars,ShowRests
|Bar
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Bar|SysBreak:Y
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Bar
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Bar
|RestMultiBar|NumBars:2|PrintOnce:N|WhenHidden:ShowBars,ShowRests
|Bar|SysBreak:Y
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Bar
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Bar
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Bar
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Bar|SysBreak:Y
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Bar
|RestMultiBar|NumBars:3|PrintOnce:N|WhenHidden:ShowBars,ShowRests
|Bar
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Bar
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Bar|Style:SectionClose|SysBreak:Y
|AddStaff|Name:"Staff-1"|Group:"Standard"
|StaffProperties|EndingBar:Section Close|Visible:Y|BoundaryTop:10|BoundaryBottom:10|Lines:5|Color:Default
|StaffProperties|Muted:N|Volume:127|StereoPan:64|Device:0|Channel:2
|StaffInstrument|Trans:0|DynVel:10,30,45,60,75,92,108,127
|Clef|Type:Treble|Visibility:Never
|Key|Signature:C|Tonic:C|Visibility:Never
|TimeSig|Signature:4/4|Visibility:Never
|Rest|Dur:Whole|Visibility:Never
|Bar|Visibility:Never
|Rest|Dur:Whole|Visibility:Never
|Bar|Visibility:Never
|Rest|Dur:Whole|Visibility:Never
|Bar|Visibility:Never
|Rest|Dur:Whole|Visibility:Never
|Bar|Visibility:Never
|Rest|Dur:Whole|Visibility:Never
|Bar|Visibility:Never
|Rest|Dur:Whole|Visibility:Never
|Bar|Visibility:Never
|Rest|Dur:Whole|Visibility:Never
|Bar|Visibility:Never
|Rest|Dur:Whole|Visibility:Never
|Bar|Visibility:Never
|Rest|Dur:Whole|Visibility:Never
|Bar|Visibility:Never
|Rest|Dur:Whole|Visibility:Never
|Bar|Visibility:Never
|Rest|Dur:Whole|Visibility:Never
|Bar|Visibility:Never
|Rest|Dur:Whole|Visibility:Never
|Bar|Visibility:Never
|Spacer|Width:3700
|Rest|Dur:Whole|Visibility:Never
|Bar|Visibility:Never
|Rest|Dur:Whole|Visibility:Never
|Bar|Visibility:Never
|Rest|Dur:Whole|Visibility:Never
|Bar|Visibility:Never
|Rest|Dur:Whole|Visibility:Never
|Bar|Visibility:Never
|Rest|Dur:Whole|Visibility:Never
|Bar|Visibility:Never
|Rest|Dur:Whole|Visibility:Never
|Bar|Visibility:Never
|Rest|Dur:Whole|Visibility:Never
|Bar|Visibility:Never
|Rest|Dur:Whole|Visibility:Never
|Bar|Visibility:Never
|Rest|Dur:Whole|Visibility:Never
|Bar|Visibility:Never
|Rest|Dur:Whole|Visibility:Never
|Bar|Visibility:Never
|Rest|Dur:Whole|Visibility:Never
|Bar|Visibility:Never
|Rest|Dur:Whole|Visibility:Never
|Bar|Visibility:Never
|Rest|Dur:Whole|Visibility:Never
|Bar|Visibility:Never
|Rest|Dur:Whole|Visibility:Never
|Bar|Visibility:Never
|Rest|Dur:Whole|Visibility:Never
|Bar|Visibility:Never
|Rest|Dur:Whole|Visibility:Never
|Bar|Visibility:Never
|Rest|Dur:Whole|Visibility:Never
|Bar|Visibility:Never
|Rest|Dur:Whole|Visibility:Never
!NoteWorthyComposer-End
Not the "single staff" solution you asked about but an effective compromise I think.