Hello,
I have forked the latest repositories available on GitHub and I have manually diffed and inserted all my changes. I have added some more code that I have not tested yet in order to refactor a couple of things and add a basic 'srchost' support, but I think that overall the design choices already discussed with Marek should be clear.

The code until now is in a single commit, in the "gsoc-port-forwarding" branch in the affected repositories:

 - https://github.com/lsd-cat/qubes-core-admin/commit/6f78a5fcf707140a6401c64a8ed9c8a47ce6e3a1
 - https://github.com/lsd-cat/qubes-core-admin-client/commit/3b02e93cba17303598db0f0ed031793c6f79dec1

Thank you
Cheers
Giulio