After trying to understand how can I use Rebex to model such a scenario, I think I found a logical issue. Let's name the external SSH server: E, the internal SSH server/behind the firewall: I, and other systems behind the firewall S1, S2, so on.
1. The firewall allows *outgoing* connections on port 22, meaning that I can connect/tunnel to E using port 22 (I -> E).
2. I can create *outgoing* tunnels to S1, ..., Sn (I -> S1, ...Sn), meaning that for each such tunnel a new port will be made available on I (which is fine).
3. E needs *outgoing* tunnels to S1, ..., Sn through I.
4. On E, I need a connection object to send commands/bind a terminal.
The problems here are related to the firewall between I and E.
P1: I need an *outgoing* tunnel from E to I, provided I can have only an *outgoing* one from I to E.
P2: Even if I can have multiple ports open on I and E, however the firewall allows only one (outgoing 22).
Maybe I missed something, but to me it seems that this cannot work.