I've had doing this on my TODO for a few years, but never managed to get around to it. So strong +1 for the idea :)
(Won't actually work because the web site isn't serving "12" URLs yet, but that's something that could probably be sorted out.)
Why not just link to /devel/ when it's a devel version? The 12 docs will be up alongside the first beta version, so it should be perfectly possible to have it do that based on information from configure, no?