For general, automatic NAT-style port forwarding: No, not really.
What you're describing is NAT64 or NAT-PT. NAT-PT is older and more limited and ultimately it was declared broken beyond hope; NAT64 is meant to replace it but it's still a draft and has not been widely implemented. It's a deceptively complicated problem. AFAIK, any Linux support for either is experimental at best. In any case, it would be too big/complicated to fit into Tomato, and the vast majority of home users are unlikely to need anything like this for the foreseeable future.
HOWEVER: in this situation, since you control both ends, you could probably use SSH tunnelling. (Assuming the IP camera uses, or can be set to use TCP, rather than UDP.)
- Establish an SSH connection from the client machine to the router over IPv6
- As part of that connection, set SSH to forward a local IPv4 port(s) on the client machine to the camera's IPv4 address and whatever ports it listens on
- Tell your browser or camera management software on the client machine to connect to the client machine itself
But the painful degree of tunnels-within-tunnels going on here make me think there's likely a simpler, better solution for you that doesn't involve IPv6.