Cool, I'm glade to see you digging into the source.
Unfortunately I'm overloaded currently with my work, but, I'll try to find some time on weekend to give you some advice from were to start, and how to build guitarix plugins. On first advice I could give, we use FAUST for most of our plugs, http://faust.grame.fr/
Experiment with this language will help a lot in the first run (and it's a lot of fun, indeed, to work with the included examples from the faust source).
We've scripts included in our source (tools directory), which will build and install guitarix plugins directly from faust sources, so you'll have quick results to play within guitarix.