Is there anything "forbidden" about making a plugin that supplements/overwrites tailor rules? Is that a thing?

Definitely not forbidden, just not a capability right now. I recall there being some discussion of this idea in the past. https://github.com/pantsbuild/pants/issues/15510

I had an idea for a cmake migration plugin, where it would parse cmakelists and do a best effort tailor Seemed pretty reasonable, except overwriting the existing mechanism that wants to run by default

I intentionally added options though so that you can disable every single tailor implementation in pantsbuild/pants

Programmatically?

No, via the options system

Ah, okay - I'll explore that when I get there. It would be nice to not have to manually migrate my cmake files