I know a verticle foregrip is a no-no on an AR pistol. But what about a horizontal foregrip(add a verticle foregrip to the side of a quad rail). This would give a "sten"-like grip.

Thoughts?

You are not the first to think they have found a loop-hole around the VG issue.  Sorry to say, there have been thousands way ahead of you and no, you cannot just move it to the side, call it a horizontal grip, and think you have outsmarted the ATFE.
The ATFE has OKed the AVG because it acts as a hand stop more than a grip, but any second grip, up, down, or sideways, makes the pistol an OAW.

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