No first-hand knowledge, since I've never bought or imported an Alu-Cab (yet). I do know a guy who's successfully imported a Troopy with a pop-top and Customs didn't care about it at all.
From a legal standpoint, they're mostly concerned with modifications to original emissions and safety equipment, but from a practical standpoint, they don't really have the bandwidth to catch every single detail of every make, year, and model of vehicle they see.
That’s the way I’ve come to understand all of the documentation as well.
While it was a factory option, Toyota Hiace and some other variants can be legally imported with a pop top roof, which leads me to believe there’s allowance for it.