Does SSH support forwarding miscellaneous ports between two firewalled
hosts? My scenario is this:

12345-| |-12345
MyPC |---------------22-| Server
6000-| |-6000

Forwarding 12345 on server to MyPC:

ssh -R 12345:mypc:12345 server

This seem to work as long as mypc can be resolved by the server.
However, it does not work when mypc is NAT'd and unresolvable.
Shouldn't it work though? Considering I initiated the connection,
shouldn't the remote server be able/willing to send the data? X11
forwarding works in this case, and with a much simpler syntax:

ssh -Y server

Shouldn't something like this work in this case:

ssh -R 12345:localhost:12345 server
ssh -R 12345::12345 server
ssh -R 12345 server

Am I missing something?