Standard agreements

For the former, the “Developer's Certificate of Origin 1.1“ in Linux’s Documentation/SubmittingPatches is pretty good. It’s less permissive than what you described --- it indicates submission under the open source license indicated in the patched file.

For the latter, the language in gnulib’s doc/Copyright/assign.future.manual is fairly clear. It allows the FSF to choose a license in the future and imposes some requirements about the nature of such a license. Other projects might not find the text easy to reuse as is, but it seems worth coming up with terms that are equally clear and thoughtful.