Yes. That's what I'm using there.
> I had hoped to plug it in project.el but I couldn't find a way, so
> I rebound C-x p f to it.
Should be easy enough: you create your own backend (rather than reusing
'transient') and override the generic 'project-files'. Then add it to
project-find-functions.
iiuc project-files should return a list of files, or can it return a completion table as well? If not the latter, then i think it won't work...
Anyway, this is the problem with the popular recommendation: just add a
thing to 'project-find-functions'. It degrades performance and changes
how project commands work too.
I don't see the relation. If anything this reduces the amount of files to search (but it's still a lot).