Thanks, Eric and Josh. I've used the ShapeShifter class from topic_tools
to build a multi-topic, on-demand proxy. It's still advertising its
topics using the '*' checksum since I really don't want to have it
subscribe to any topics before it starts advertising them, but that
seems to work all right (and should be fine as long as you don't have
mismatched messages [?]).

http://www.ros.org/wiki/proxy

I constructed a message mismatch -- where the sender and the receiver
(across a repeater) had different message definitions -- and tested it
using rostopic. The connection was dropped immediately.

Ken