I have commited some complete examples in
http://gnuradio.utah.edu/svn/gnuradio/trunk/gnuradio-examples/python/channel-coding/
You can now see how gr-trellis can be used for implementing:
generic convolutional encoding/decoding
serially concatenated coding (through an interleaver) and
decoding using two Viterbi algorithms (no soft output yet…)
Documentation can be found in
http://gnuradio.utah.edu/svn/gnuradio/trunk/gr-trellis/doc/
Enjoy,
Achilleas