Hi,
I think if we don't want to change back our configuration (batman-adv on vpn with its mtu-problems etc.) the simplest way would be connecting a usual router to the wrt54g.
I don't think the VPN is the problem here but the fact that you can't bridge an interface running in adhoc mode. Ad-hoc is "lacks" a field in the wifi header which makes bridging impossible. However, I agree with your proposed solution - another router connected to the Ethernet of the wrt54g can solve the issue.
Regards, Marek