Hi Joao,
At least the idea of using a VPN doesn't require any changes to libmapper to implement. If you are running a VPN then all computers on the VPN should be on the same subnet and be able to communicate, and it should "just work." It's not guaranteed that multicast will work, however -- this I can see being the main challenge. You would have to try it.
Johnty what VPN did you use?
If you have control of both computers over the internet, or perhaps a single server that two computers can connect to, then maybe one you could use is OpenVPN.
I do wonder if there's a way to forward things over SSH, it tends to have nice forwarding tools for specific ports but I don't know about multicast.
Steve