This sounds doable, actually. You'd want to pre-install (say, from a local wheel) a matching dummy dependency where the metadata claims that it's the right version of whatever package (so the installer will just see that the dependency is "already satisfied" and skip it), but the actual implementation code just exposes a hook to your mocking system.

Doesn't work if version resolution decides to upgrade or downgrade your installed package, so you need to make sure the declared version is satisfactory, too.