No, tf is copied across as it is. Also, our system has no message
introspection capabilities, so changing fields inside a message would be
difficult. Our policy so far has been to avoid TF name clashes across all
systems. This is certainly annoying in multi-robot scenarios, but I haven't
seen a nice, completely robust solution yet.

Hugely annoying, yes. The gold standard for ROS multimaster is definitely when each individual robot looks exactly like a non-multimaster robot, and can be added to the multimaster system with no modifications to its underlying launchfiles, topics, diagnostic outputs, TF frames, etc. But that obviously requires some sophistication at the robot boundary to handle translation. And it gets interesting in sim, for.... well, a variety of reasons.

No one has released an end-to-end solution yet, but we're working on it too, of course. :)