Is there a way to set a glyph to a specific position on a variable axis through OpenType? E.g. if I have a width axis and want to set a glyph to a specific width when followed by another glyph. Is there a way to write this in the OpenType features somewhere?
No, but others are thinking along the same line. For example:
Bummer for no per-glyph axis position, but that link is very interesting! Thanks for sharing. I had a similar conversation around the same time with Eliyezer Kohen just as variable fonts were being seriously implemented — Had hoped at that time there would be support for inter-run variation but looks like that’s still not happened. Hopefully they continue to discuss and pursue 
What exactly are you trying to do?
Couldn’t that be done with contextual GSPO and variable value records?
Maybe? I have an Arabic font with a return yeh which I would like to have line up exactly underneath various letters. There’s a TAIL axis already which lets the user select their own length, but by default I would like to extend/retract the tail as necessary per letter. For example, lam+yeh would need a much shorter tail than ayn+yeh.
What are variable value records?
Now I understand. That is not possible.