Hi @hugetim ,
I have no advice w/r/t iframe …
However, 100% you can use a custom domain name in GH Pages
I am using this on a few sites Here is one, just to prove it
Basically using a GH theme, but my custom domain (FWIW, my registrar is GoDaddy)
Happy to answer any questions, but the GitHub pages are pretty good.
Cheers,
Tyler