The "test*Streamer" test programs read from a file. Can I modify them so that they take input from a MPEG encoder instead, so I can stream live (rather than prerecorded) video and/or audio?

Yes. The easiest way to do this is to change the appropriate "test*Streamer.cpp" file to read from "stdin" (instead of "test.*"), and then pipe the output of your encoder to (your modified) "test*Streamer" application. (Even simpler, if your operating system represents the MPEG input device as a file, then you can just use the name of this file (instead of "test.*").)

Alternatively, if your encoder presents you with a sequence of frames, rather than a sequence of bytes, then a more efficient solution would be to write your own "FramedSource" subclass that encapsulates your encoder, and delivers audio or video frames directly to the appropriate "*RTPSink" object. This avoids the need for an intermediate 'framer' filter that parses the input byte stream.