Other than the section configuration guide covering IPv6 (here), I don't know of any specific document detailing IPv6 support for the FWSM. Cisco does not generally publicly publish roadmaps outlining future products enhancements.
Product-wise, while the FWSM has not been announced to be end-of-sales I would expect the majority of development effort by Cisco in this arena to favor the new ASA Service Module. Note the Q&A for the ASA-SM here. The ASA-SM does support IPv6 failover. See the configuration guide.
