IF the receiver was built as an "other" then it's not a pistol, which the OP specified.
I haven't seen anything about a handstop being a problem on a PISTOL as long as you can't grip it vertically. I'm using B5 "barrel guards with a forward projection."
I'm not the ATF and I haven't stayed in a HE but I get the impression that if it's no deeper than a one finger engagement it's ok. Two fingers and you are skating on thin ice.
Then there is the issue with it being a 10.5" barrel typically. Nobody is going to get full extension with a locked elbow on it like a 3Gunner. It's going to take a completely different hold. If you are holding onto the barrel not magwell then getting your hand out in front of the stop isn't likely. And if you don't have your hand on it when you pull the trigger getting your off hand shot could happen.
A handstop is just a reference, not a guaranteed safety device.