I’ve found a few workarounds, nothing very satisfactory, but this one below involves creating a custom notehead. How would you go about this?
Other options I tried involved nudging the notes in Engrave mode, but that makes a mess of the accidentals. Perhaps there’s a better way of notating it? I basically want the arpeggio to end on the grace note and then quickly down to G#.
Your help is highly appreciated!
“What is the proper way to achieve this?”
Are you speaking about engraving or playback?
Why not notate it in the most normal way?: clear for the player and correct in the playback:
Or if you want really that look (which I think is less clear for the player), you can use a tuplet 9:8x (and hide it) for the upper voice (you need force duration to notate the half note inside the tuplet), and change the size of the A to Grace note in the Properties:
Nice! Are you a keyboardist? Would your first example be the most clear to you?
Yes and yes 
Aaaaaaah … good old Sibelius days when we faked everything and their ancestors with tuplets!
You rock!
