Closing, after looking a bit more into this, @simoncozens has done so much work in fontFeatures that looks like we'd have to reproduce that it's not worth the effort — aka: it's not a simple quick drop-in to use just fontTools (main goal here was to reduce the number of dependencies).
Will reopen if someone chimes in to tell me that I'm wrong and it is a quick drop-in.
Closing, after looking a bit more into this, @simoncozens has done so much work in fontFeatures that looks like we'd have to reproduce that it's not worth the effort — aka: it's not a simple quick drop-in to use just fontTools (main goal here was to reduce the number of dependencies).
Will reopen if someone chimes in to tell me that I'm wrong and it is a quick drop-in.