Can I put an attachable forgrip on my pistol?

I own a Kel-Tec PLR-22 and finally found the forend with the underrail, so I’m hoping to tacti-cool it up now that I have rails available.

One of the first things I wanted to get is a detachable foregrip because the gun is pretty awkward to hold. It’s too forward-heavy to aim with one hand, and I don’t like holding the magazine to steady it. However, I’m hearing two different sides on whether or not this is legal.

One camp says that, as defined by the ATF a pistol “is designed to be fired by one hand” and that adding a foregrip turns it into an AOW that’s illegal without the 200 dollar tax stamp from the ATF.
However, the other camp says that only applies to permanent modifications and that since a detachable grip isn’t permanent, it doesn’t count under that rule.

Which camp is right? Can I get one, or should I just sigh and use the extra space for a laser sight instead?