Does pants support an equivalent of bazel's

git_repository

which will clone a repository and apply patches/inject build files at target execution time?

Not out of the box. Can you share an example how this is useful? (Me being curious;) )

we use it for pulling 3rdparty repositories mostly. Normally you would add a git submodule and check in your added BUILD files, but this won't work with a big monorepo codebase so you want them pulled when needed as a dependency. Injecting the BUILD files or patches also allows you to avoid modifying the 3rdparty repo