tar commit header names

I'm entirely ignorant of pax header stuff, but would it be possible to 
have the pax header specify which project this commit is from (which has 
to come from a git-archive option, clearly). Also, could 
git-get-tar-commit-id have an option to look for more pax headers beyond 
the first? (I'm building tar files by appending multiple archives for 
different projects from git-archive, and I suddenly find the embedded 
commit id is helpful, and surprisingly inaccessible in this context.)

	-Daniel
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