The one place I knew to offer custom focus screens, KatzEye, doesn't appear to support full-frame cameras. They probably figured that the Canon screens have the majority of the market covered, and it's not worth investing the the few people who want something custom. Either that, or their screens don't meet the quality of pro users. No idea which. I've read glowing reviews, so I'd guess it's the former.

I crop 90% of my shots to 5:4 or 4:5 (ie: 8x10), so I can understand the desire to have crop line in the viewfinder.