You mean this:
http://www.hkparts.net/shop/pc/HK-Ge...te-107p217.htm
?
It'd be hard to argue that's a frame or receiver or barrel. It does meet the vague definition of a "receiver" though.
However, the lower receiver of a scar is a receiver. Sounds absurd, I know, but it fits the legal definition of a receiver and, surprisingly, it's also called a receiver. Who would have thought! But engraving a non-metal part probably isn't legal, which is why you see polymer-framed pistols have a metal plate that bears the serial number in their frame. If a SCAR lower doesn't have metal surfaces to use for the engraving, it wouldn't be usable for that purpose.
Basically, Scott, you may be right, but not for the reasons you've listed.