Ah yes, I just bumped into something similar!
A couple of suggestions:
- Swap the a machine executor, where you have root access. This is free for the time being, and has been for a year or so, but the price may change in the future
- Still on the Docker executor, use Docker Compose with three containers: one system that publishes on port X, one system that consumes on port Y, and a proxy to do the translation. There are no networking restrictions between containers, so this should work.