Hi Bart,
I understand what you are saying, and agree in principal; however, the individuals who created those fonts made a useful contribution, since at the time it was the only way to allow those symbols to be added to a score.
Thank you for the kind words about the slur object. If you have looked at the API, you'll see that an object can be created that draws lines, shapes and text on the staff. The "text" includes note shapes from NWC's system font, so it would not be that difficult to create an object that creates the rhythm indication you found. However, I've had another idea for creating what you asked for, that you can see in the following clip:
!NoteWorthyComposerClip(2.749739,Single)
|Clef|Type:Treble
|TimeSig|Signature:4/4
|Note|Dur:8th,Grace|Pos:7|Opts:Stem=Up,Beam=First,NoLegerLines,Muted
|Note|Dur:8th,Grace|Pos:7|Opts:Stem=Up,Beam=End,NoLegerLines,Muted
|Text|Text:" = "|Font:PageSmallText|Pos:7.5|Wide:Y|Justify:Center
|Note|Dur:8th,Triplet=First,Grace|Pos:7|Opts:Stem=Up,Beam=First,NoLegerLines,Muted
|Note|Dur:8th,Triplet,Grace|Pos:7|Opts:Stem=Up,Beam,NoLegerLines,Muted
|Note|Dur:8th,Triplet=End,Grace|Pos:7|Opts:Stem=Up,Beam=End,NoLegerLines,Muted
!NoteWorthyComposerClip-End
As you probably have guessed, they are just muted grace notes with no ledger lines. If you were going to use this approach, you would probably want to put them on a separate "ornament" staff, surrounded by invisible rests of the desired duration.
That doesn't mean I am giving up on the notion of a user object approach, but I thought it was worth showing there is more than one way to skin this particular cat.
Mike
[Edit: Before anyone says it; yes, I agree that the notes are a little on the large side.]