Apologies if I'm being a little dim.
From the http://tomatousb.org/doc:build-features-cross-reference it says ipv6 is included in the 'normal' VPN build, which I have downloaded (tomato-K26USB-1.28.9054MIPSR1-beta-vpn3.6.trx) for my router, which is an ASUS WL500gpv2. It also says there is no GUI support. Elsewhere, I have seen there is GUI support, found under the Basic options. This doesn't exist in the build I have.
For example, this page http://www.broadbandreports.com/forum/r25551608-DSL-WRT54-Series-IPV6-Support shows the ipv6 settings, for a WRT54, which is using the same R1 CPU and appears to be using the same build number. It also mentions settings are to be found under the Basic options in this thread http://tomatousb.org/forum/t-285632/ipv6-features.
Is there something I need to do to enable access to this settings page, or is it simply that I've downloaded the wrong build