I like this idea, but I don't think it's sound to consider something GPL-licensed because the author checked a box or accepted the TOC. I doubt that has any legal significance.
I agree that accepting a one time ToS agreement is hardly binding. That's why I suggested printing out a message with a Git hook saying everything you push is going to be GPL, it's a bit harder to claim ignorance when you see that message every time. We're already in the gray area since there's thousands of packages with no license explicitly listed, and I don't think it's sane to suggest removing those.
Wouldn't it make more sense to use a mandatory two-line header like below?
This switch to Git is already going to cost us a lot of maintainers who simply don't feel like learning it, I can't say I'm in favor for imposing any more requirements. I'm also not really a fan of littering my PKGBUILDs with static text either. Listing the current maintainer and contributors is already less than ideal (with the move to Git, I guess we could stop listing the contributors inline since they're in the logs).