Hi I am building element42's adapter as well. I have not done a faithful reproduction, but rather have built the basic interface, and am now playing around trying to get the best setup.
My goal is using the Ultraracer controller with this adapter. This is basically a NegCon with additional L2, R1 and R2 buttons.

Currently open problems (maybe somebody can help):

1) replicate the PS button somehow; this appears to be possible, according to this post and comments

2) somehow try to map the analog buttons to the analog L2 and R2 inputs. Does anyone know how the reportDescriptor is supposed to be set up for this? As of now I have L1 and I buttons on the negcon mapped to Z and Rz, but the PS3 does not like this setup (works fine on my mac though)

3) determine setups for a few games, then save them to the microcontroller's
flash memory and allow the user to cycle between different setups.

Any input is welcome!

CRP