Aren't you reimplementing GH_TAGNAME with this?
I think this is fine, see Example 5.14. Using USE_GITHUB to Access a Commit Between Two Versions in the Porter's Handbook.
I think that we should use @sample here (see 8.6.9. @sample file [file] in the Porter's Handbook).
Yeah, I always thought of DISTVERSIONSUFFIX=-gWHATEVER as a "higher level" wrapper around GH_TAGNAME
Do the @ things work with PLIST_FILES (w/o dedicated pkg-plist file)?
Yes, of course. PLIST_FILES is basically just appended to TMPPLIST. Just make sure to quote the entry like "@sample etc/intel-undervolt.conf.sample", so that make(1) treats it as a single word and it's added as one line to it.
Updated with suggestions
This looks ok.