ReaPack source is here (it's C++)
https://github.com/cfillion/reapack
A basic extension plugin example is here:
https://forum.cockos.com/showthread.php?t=93355
Out of interest, what do you want to do exactly ?
Maybe it's sufficient to go the script route instead of (more advanced) C++ extension plugin route ? Lots of graphic things are also possible by scripts as can be seen by various scripters here.