Hi:
Our users don't care about IPv6 as long as they can get into Internet without issues... of course your mileage is different.
About RAM: We don't have any RAM issues, KERNEL_SWAP is not set, so any RAM shortcomings would hang the devices, still all devices (more than 40) are working without issues. Of course they don't need to have any routing tables, just a gateway, and again, YMMV.
I think snapshots could work for you and they do have IPv6 enabled. I don't know if you think you need Luci too. The fact is, 4/32 devices are very little things now, and, if you are looking for a recent openwrt image fully-featured to work on those devices, you are crearly asking for too much.
So, I think you have three options:
a) If you don't want to build your own image, and want to have a recent, full-featured image working, a more beefy device should be chosen, as you don't need to replace many devices but only one.
b) If you don't care about luci, snapshots for tiny devices will probably work fine, they do have IPv6 enabled, no need to build custom images
c) If you need to build your own custom image, you could try that diff as an entry point, of course you'll need to make your own changes -as putting IPv6 on.
Hope this helps