In NWC, is it possible to have, for example, eight sixteenth notes (stem up) beamed with the top beam across all eight notes, and the lower beam stopping between notes 4 and 5? If not, is there a hack to do it?
Yup. There is an available note property called "Beam group start". Select the 5th 16th note, press <Alt+Enter> to access the property dialogue, and tick the "Beam group start" check box.
!NoteWorthyComposerClip(2.751,Single)
|Note|Dur:16th|Pos:-1|Opts:Stem=Up,Beam=First
|Note|Dur:16th|Pos:-1|Opts:Stem=Up,Beam
|Note|Dur:16th|Pos:-1|Opts:Stem=Up,Beam
|Note|Dur:16th|Pos:-1|Opts:Stem=Up,Beam
|Note|Dur:16th|Pos:-1|Opts:Stem=Up,Beam,BeamGrp
|Note|Dur:16th|Pos:-1|Opts:Stem=Up,Beam
|Note|Dur:16th|Pos:-1|Opts:Stem=Up,Beam
|Note|Dur:16th|Pos:-1|Opts:Stem=Up,Beam=End
!NoteWorthyComposerClip-End
I should mention that beam groups is one of the new features added in the latest version (2.75a). You can see a history of program changes and additions here (https://noteworthycomposer.com/info/history.htm).