I had a quick glance yesterday and problem to put the version before 3.1 is that the title of a guide comes from its .textile file.
But, regarding the concern you express above, outdated guides should simply not exist in an ideal world. If it happens, it is an overlook by someone not updating the docs in a patch that should. Still happens, but I hope it happens less and less as time passes.
So, the Rails version as we have in the home page could be informative, but I would not put a moving version with that purpose, really, it would be even more difficult to control (are ALL updates done here?), and it would implicitly say the scenario where outdated content happens is expected. No, that scenario is forbidden in all the parallel universes and the ten thousand things.