Johnso,
I cannot find the reference but I think you can only deviate from DH and get PS on a mainline flight. It used to be (I do not know if it is still this way) that you could not be DH'd on DCI. One time I had to do a double DH to get back to base at the end of a trip when there was an RJ non-stop. I deviated and nonreved back on the RJ. You can still deviate but just not get the PS.
Denny