Hey everyone,
just in case it helps someone in the future ; I found the solution, which was in a detail I forgot to tell about ; it's a vmware installation.
My set-up was OK, the TAP VPN was up, and it was forwarding L2 trafic, however the vmware host simply discards any packat with a mac "not from the guest", which makes it impossible to have something like an ARP-proxy (or Layer2 vpn) on a vmware guest
Solution is to allow "promiscuous" on the vswitch (altough I don't need promisc mode at all, I just need less paranoid enforcement of the MAC filtering)
I tried disabling the other MAC-related option, but it did not work. Only works when allowing "promisc".
Hopefully this helps someone someday