I have to admit I've always assumed a Hartman Mask would be useless with such wide angles, but could be wrong?
I just found an interesting idea which JohnG's response got me hunting down - perhaps this is exactly what you mean John? :
http://www.canon.com.au/products/slr...s350dblackbody
(Dioptric Adjustment Lens to go on the eyepiece of the SLR)
EDIT: My mistake... they don't me 2x magnification, it's just correction for those who wear glasses etc.