using transparent gateways to the launchpad platform?

[Denys Duchier]

My students cannot access launchpad through lp->bzr+ssh because they are
using a parc of machines for which no NAT translation is provided (and
never will be).

I am thinking that, if, at the time of getting a transport, bzr could
automatically substitute bzr+ssh://XXX.launchpad.net/... with
bzr+ssh://SOME-LOCAL-GATEWAY:PORT-FOR-XXXX, then I could set up
appropriate port forwarding on SOME-LOCAL-GATEWAY for the necessary
launchpad targets.

So here is my question:

What would be the simplest way to perform this translation
transparently, so that my students can continue using the usual lp:
URLS?

Cheers,

--Denys