i can try to write something.
already posted few weeks back some tutorial on the basic of where do u need to go inside admincp to create new product (
[How-To] Simple hack development stages)
though i didn't go into too much coding (if any), only gave "quick info" about mysql query code lines & some usefull arrays, so you got indeed some point there.
maybe i'll try to write some tut showing information about the "default sig per usergroup" modification whitemike & i wrote @ the time.
for all other coders, don't let me stop you :p
i think indeed that the more tutorials explaining plugins posted here, it can sure help one understand the point of view & from where to start...
so anyone with some free time is welcome to post his guide as well.