Can anyone tell me if I can use a 10" Fortis Night Rail with an ASR mounted Omega on a 10 1/2" barrel?

You may find that rail protruding a bit past the muzzle threads so with the suppressor mount it may interfere. Unless the inside diameter of the rail is large enough to accept the outside diameter of the suppressor.

The thumb rule I follow  to maintain clearance between the suppressor and the rail is to use a .75" difference in rail length to barrel length. This way I get just the right amount of clearance from the threads for my suppressor mount and the rail.

I *think* you'll be ok as the threads on the ASR are a little further forward than the rear of the muzzle threads.