I ended up doing it a different way in the end.
I used your mirroring script to mirror the repos I'm interested in, and on those repos I set it to use an external gitlab-ci config file. I knew you could use includes in your gitlab-ci.yaml, but I didn't know you could just point gitlab to an external config file completely.
This way I don't have to change the actual repo itself, whenever it is sync'ed by the mirror script, the build kicks off. It's good enough for what I wanted 🤷♂️