Yes it is possible, there is no link per say depicting webvpn as a client source and L2L tunnel example, but look at this link, same principle the difference is the client is using Cisco VPN client and can access resources in PIX3.. the L2L termination is between PIX1 and PIX3, vpn client comes through PIX1.
You already have exsiting L2L tunnel between A and B sites. Now you can simply use same-security-traffic permit intra-interface and add your WebVPN pool network to your L2L tunnel interesting traffic nat exempt acl.
We have configured the outside and inside Interface with official ipv6 adresses, set a default route on outside Interface to our router, we also have definied a rule , which also gets hits, to permit tcp from inside Interface to any6.
In Syslog I also se...