I have small hands too and the backstrap as is is very comfortable - the HK45 is more streamlined and thinner in the grip than the USP45 which is why it has dedicated magazines - what is your issue with the grip as is, are you not able to reach controls without shifting? Trigger finger placement? Safety/decocker?
Mag release, and slide lock/stop both are great for me on my HK45 and the HK45C.
Full disclosure, I will say that with my smaller hands, I cannot in any form use the manual safety/decocker without seriously straining my grip to do so, on the HK45 series as well as the USP's - However, I am not a manual safety type and prefer no external controls, so all my hammer fired HK's are either factory LEM or converted to LEM
People don't understand for the most part as I watched IV8888's video with John Lovell basically making fun of people who say guns don't fit their hands, but they are thinking the opposite effect (hands are too big etc) - they don't think about people with smaller hands not being able to easily manipulate controls.
I got so excited for the Beretta M9A3 because of the thinner Vertec Grip, and I was going to convert it to G version! Finally held it in store, and I was crushed - I could barely get to the mag release, but I could do it, but the dealbreaker was I literally had to use my support hand to decock the pistol, I could not get my thumb up high enough to comfortably decock, so its unfortunately a no go for me - I don't want to be the idiot with the cool gun in his safe that he can barely manipulate controls on. Same with CZ75's, that DA trigger pull I can barely get my finger on.
I have some hope for the Mark 23, even though it is a larger pistol, USP's don't seem to bother me either, thank God for HK and their paddle mag releases.