> changing one entry in the history will ripple sha1 hash changes throughout the whole tree.
This is absolutely true. VCS bridges like git-svn and git-hg store the meta information in each commit's comment. The git-filter-branch script could, as you say, add a "Original-Commit: CF22A1..." line.
Not sure it would be worth it though -- I can't imagine the number of people using the FSF's tree would ever be very high (witness Gnusense).
The FSF is has a decent point, but the way they're making it is simply bizarre. It seems like they're trying to appeal only to their most fanatical supporters and discouraging everybody else. If they took a more mellow, community-oriented, and sustainable approach, people might actually listen. As it is, I bet this effort will go the way of the GNU/Linux appendage: lots of discussion, fizzle, and forget.