i have seen posts before about people who kept their content separate from the actual site layouts and what not, using 2 different repos. I don't know how they did it (not great at git) but I believe it is possible to an extent. I think that was probably in the old forum that got deleted though.
But if you are looking to do that on the fly with multiple different repos that are not from the same owner it might not be viable.
I wouldn't be surprised if there is a way via the GH API to get the readme for a repo, then you could use JS to pull that in and then use a markdown library to make it html. I do something similar with GH issues, using that as a changelog on one of my sites, works really well.