So, Builder seems to have the most complete implementation of BlankSlate, short of Ruby 1.9's BasicObject. The problem is, this is part of Builder, and still inside the Builder gem.
It's especially frustrating, because the Builder source (lib/builder/blankslate.rb) seems to acknowledge that there should be a separate gem. But the only reference I can find refers to onestepback.org's gem repository, which isn't working.
So I built my own. I'll try to keep it up to date with Builder. The first three parts of the version number are the Builder version; the last part is my revision.