Joseph Dickerson , Dec 21, 2011; 02:44 p.m.
Anything that moves the ground glass out of its current position (for example, placing a fresnel on the lens side) will throw your focus off.
The only thing I can suggest is to be careful with the Sinar fresnel. Mine is quite old and still not too scratched up. Besides, it's nice to be able to remove it when you want. It also makes adding masks when shooting other formats extremely easy.
You might contact S. K. Grimes and see if they have any suggestions.
You might also post your query on the Large Format Forum if you haven't already.