Yes, hopefully only 1 message is sent over s2s connection, with a xep33-element to indicate the addional addresses. But only if the destination server also supports XEP33.
1. Participant A writes in the MUC room.
2. mod_muc_room, instead of routing a packet for each other room participant (B and C), it calls:
ejabberd_router_multicast:route_multicast(From, Server, JIDs, Packet)
From = jid() % JID of sender: A
JIDs = [jid()] % list of JIDs of the other room participants: B and C
3. ejabberd_router_multicast (implemented in another patch), checks what are the servers of B and C: server-2. Then it checks if server-2 supports XEP33.
4.a If server-2 supports XEP33, ejabberd routes 1 packet to server-2, indicating which addresses to deliver: B and C. It will be server-2 who sends packets to B and C.
4.b But if server-2 doesn't support XEP33, then ejabberd routes 2 packets as usual: a packet for B and a packet for C.