On somewhat more mature consideration, I believe that this problem, to the extent it is a problem, simply comes from misplacing the operation of inserting the revision number into the file. Currently, the operation is done at the point of exporting asdf.lisp into a tarball. In my previous email (unfortunately only in a footnote), I proposed a technique for moving the revision insertion from export to tag application. I believe that if the makefile (or some shell script) is modified in the way I propose, everyone should be happy.
This modification would make no additional work for Gary, and would lift the burden of massaging the source file from everyone running ASDF from a git checkout. It would have the added benefit of removing 16 lines of lisp source that do fussy string manipulations to set *asdf-revision*. This seems like a good trade-off --- a few lines of shell script or makefile provides an easy operation for Gary (and simplifies the export), and saves work for the development community.
Would this be an acceptable solution?