Very nice indeed.
I looked into something like this a while ago, but as you mentioned lack of code/details, getting a sensor, plus i wouldn't be able to write the firmware from scratch either. Ended up with a CM Recon mouse, programmable with a handful of decent buttons (for me)
If it's a project you want to build so that the community can either duplicate or adapt. i'd keep it pretty simple.
- teensy with supporting code/firmware for a specific sensor, wheel & how ever many switches you want to throw at it. Well documented firmware for where all the variables are that might need adjusted for their given setup
(at this point it could be used as a converter to swap sensors or add functionality to an existing mouse)
i'd jump all over it in a heartbeat to be able to do this much of it, even buy a mouse to scavenge the sensor out of, if they are that hard to source.
To create more of a polished product to be produced is where the difficulty & fewer selections come in.
As far as shell, i have no idea what you have in mind & i'm sure some of the members of this great community would be willing to lend a hand. One thing i did find when i was looking before was this adjustable top shell for a Logitech M100 mouse. The top shell is very interesting and very unique in many ways, just wish it wasn't created for such a basic mouse.
designers page:
http://www.pyottdesign.com/statial01.htmltop shell @ shapeways:
https://www.shapeways.com/product/T4C3TT6KM/statial-adjustable-mousemouse:https://www.logitech.com/en-us/product/mouse-m100
Anyway, Congrats! And i'll anxiously keep an eye on the project.