Maybe we could add RESTRICT=srcdist which would cause ebuilds to save
their distfiles in a separate directory controlled by PORTDIR_NODIST or
something. If the variable is unset then it's business as usual.
I was going to suggest similar.
Though I hadn't thought about it at a global RESTRICT level, but I had envisioned that potentially, the redistribubility should be a property of a source file, not of en ebuild.
For example, if there was something such as java, which would likely be mirror-restricted, it the java binaries themselves would be mirror restricted.
However, if there were any tools/patches provided by gentoo and also in src_uri, marking them as no-mirror seems counter productive.
Just the RESTRICT based interface seems more obvious, where as spicing SRC_URI values properly could be nuanced and complicated.