I got my WRT54GL working just fine with IPv6--all I had to do was use the most recent 'VoIP' build (14896) which weighs in at 3.61MB. Though according to their chart the K2.4 versions are supposed to have IPv6 support in the 'STD' build, but obviously didn't. If you're using a router that supports the K2.6 version you can get it in the 'STD-Nokaid-Small' build which is also under 4MB.
Of course, there's the problem that it doesn't include ip6tables, ping6, or any other useful tools. Which makes it really a pain to configure and diagnose for things like 6to4 tunneling. Oh, and also that IPv6 multicast packets can't pass through the shitty MAC address translation if you're using a DD-WRT router in client bridge mode.