I found the linear variable ND filter. It's one of these:
http://www.edmundoptics.com/optics/o...plitters/41960
It's metalized so acts as a variable reflector. Just dump the reflected beam into a beam stop. Max ND is 2.0 so you'll get 100:1 attenuation at the high end. You could stick a 100% mirror or a beam block at the end if you need 100% attenuation.
Make me an offer if you want to give it a try.