@JonFair I was casually browsing the forum yesterday and saw this post. I had completely forgotten about the existence of a Note Tracker in GPScript, thanks for the remainder 
Looked into it and it helped me solve a nuisance in a channel-aware octaver scriptlet I wrote. To prevent stuck notes when changing octaves while holding down keys, I used to send an “kill all notes” on a channel if the octave on that channel changed. That worked alright, but there are situations you want the held notes to stay on.
After reading this post I realised I could actually track the notes without having to write a tracker myself. Then I can send NoteOff events to only (and all) the notes that need to be released in a channel-aware manner. That was quite handy. See the octaver scriptlet where I implemented this on GitHub.
I have even seen an older related topic today, Transpose with Note Persist (Scriptlet), so I guess this may very well be a recurrent use case