So, a little more on the matter of joining. I am currently the second newest sculptor working with IWM (Dak just became the newest nearly 2 years after me).
I had two full miniatures drawn up in CAD (Vulture III and Thor II). I first proposed the Vulture, and was ultimately knocked down in lieu of the current (and beautiful) mini we have today. The Thor II was accepted as an online exclusive. My next 5 minis were all released as online exclusives (Arion, Arctic Wolf II, Chimera, Pendragon, Centurion Resculpt). It was only this year that I started submitting minis for the quarterly releases and cons (Centurion Omni, Roadrunner, Hound, and Gravedigger). Next year, I am only scheduled for 2 quarterly releases thus far, though I might submit a few Online Exclusive proposals, which may or may not get knocked down.
Don't expect to make big bucks in the process, but at their core, everyone at IWM are still BattleTech fans. Give an earnest submission, and they will definitely consider it.
I personally would recommend that you try and do a mini that is requested from Fan Financing, maybe something nearer the middle of the list. Give them detailed images, and they will consider it.
Best of luck!