> With that being said, my question remains:
> Is it possible to filter the messages before they are actually sent?
> And if so, where do I have to modify ros?

The answer to your question I believe is no. We were discussing this
feature for ROS 1.3 ("polled topics"), but it may not make it into the
next development cycle depending on how quickly we can deal with some
other high-priority items. The idea with polled topics is the scenario
you describe -- the subscriber pulls messages from the publisher as
needed.

 - Ken