I wrote this, needs to be added to a page somewhere:
The robotpy installer pulls roborio wheels from the wpilib artifactory repo (see https://github.com/wpilibsuite/docker-images/blob/main/cross-ubuntu-py/pip-aarch64.conf ... Peter can set you up with a token). The wheels have to end in linux_roborio.whl.
Just publish your x86 wheels to pypi like normal python project and the robotpy installer will pull those for simulation.
Additionally, you'll want to publish the package name in the wpilib docs, there's a table with python package names.