On my side, I have no plans to add what you need, so patches are very welcome. I can help providing indications on how to get things done if you have time to work on this. So I would suggest you fire at one front at a time.
I would start by adding the assymmetric tunnel ID allocation that you need, which should not be too complicated to add. You have to extend the netlink interface that we have on gtp to support this.
Thanks Pablo for the kind response, I will study the docs and the gtp module and try to propose some patches. Rgds, Vicent