An "Assault Pistol" defined by HRS is a semi-auto with a detachable magazine PLUS two or more of the "evil features" that define "Assault Pistol".
Therefore, if it is 12" or less, weighs less than 50 oz., has no barrel shroud (fore grip), and no threaded barrel, the AR type pistol build would only have a single feature, a magazine that attaches outside of the pistol grip.
As such, it is a pistol which in Hawaii is limited to 10 rounds if the magazine is detachable.
AR pistol builds typically have welded magazines to exclude them from the "Assault Pistol" definition because doing so negates the detachable magazine feature. This in turn allows every other feature to be utilized without restriction. It also allows 50+ round drums if one is inclined to do so since the 10 round limit only applies to detachable magazines.
As for the requirement for welding by HPD, it should be noted that the ATF made a decision concerning the Olympic Arms OA-96 regarding it's inability to accept a detachable magazine. A copy of the email can be read at the end of the following site:
http://www.quarterbore.com/ar15m16/oa93.htmKnowing Firearms Division however, I'd expect to either wait there for the whole day or be flat out denied in the first minute. After that is in a lawyers hands should anyone really want to go there.
Incidentally, the OA-98 was manufactured specifically for the "Assault Weapon Ban" that has sunset. A 4" barrel would make it a Hawaii legal detachable magazine AR pistol (Less than 12" in length).