There are a couple of ways, probably the easiest is to place a hidden octave shifted clef at the start of the 8va section and another non-shifted clef at the end. The other alternative, arguably the better one, is to place a hidden instrument patch with a 12 semitone shift at the start of the 8va section and another with a 0 shift at the end to restore things.
E.G.
!NoteWorthyComposerClip(2.51,Single)
|Clef|Type:Treble
|Instrument|Name:"Acoustic Grand Piano"|Patch:0|Bank:0,0|Trans:0|DynVel:10,30,45,60,75,92,108,127|Pos:11
|Note|Dur:4th|Pos:-6
|Note|Dur:4th|Pos:-5
|Note|Dur:4th|Pos:-4
|Note|Dur:4th|Pos:-3
|Bar
|Note|Dur:4th|Pos:-2
|Note|Dur:4th|Pos:-1
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:1
|Bar
|Clef|Type:Treble|OctaveShift:Octave Up|Visibility:Never
|Note|Dur:4th|Pos:-5
|Note|Dur:4th|Pos:-6
|Note|Dur:4th|Pos:-7
|Note|Dur:4th|Pos:-8
|Bar
|Note|Dur:4th|Pos:-9
|Note|Dur:4th|Pos:-10
|Note|Dur:4th|Pos:-11
|Note|Dur:4th|Pos:-12
|Clef|Type:Treble|Visibility:Never
|Bar
|Note|Dur:4th|Pos:-6
|Note|Dur:4th|Pos:-5
|Note|Dur:4th|Pos:-4
|Note|Dur:4th|Pos:-3
|Bar
|Instrument|Name:"Acoustic Grand Piano"|Patch:0|Bank:0,0|Trans:12|DynVel:10,30,45,60,75,92,108,127|Pos:11
|Note|Dur:4th|Pos:-9
|Note|Dur:4th|Pos:-8
|Note|Dur:4th|Pos:-7
|Note|Dur:4th|Pos:-6
|Bar
|Note|Dur:4th|Pos:-5
|Note|Dur:4th|Pos:-6
|Note|Dur:4th|Pos:-7
|Note|Dur:4th|Pos:-8
|Instrument|Name:"Acoustic Grand Piano"|Patch:0|Bank:0,0|Trans:0|DynVel:10,30,45,60,75,92,108,127|Pos:11
|Bar
|Note|Dur:4th|Pos:-2
|Note|Dur:4th|Pos:-3
|Note|Dur:4th|Pos:-4
|Note|Dur:4th|Pos:-5
|Bar
|Note|Dur:Whole|Pos:-6
!NoteWorthyComposerClip-End