For cheap access points, we run OpenWRT on something like a 32M/8M
WRT54G-TM, and there's never been a problem with memory, even after
adding somewhat piggy (for embedded) stuff like ntpd. Of course, the
normal platforms are a bit more cramped.
It's apparently very easy to add IPv6 to OpenWRT, and you can opt to
include or exclude things like a web interface. It's fairly competent
and can support things like multi-SSID. Good place to start if you're
used to a UNIX shell environment and Linux.
Anyways, the point is, a lot of the heavy lifting has already been done
to make multiple IPv6 firmwares for many of these devices.