I was under the impression that the BATF considers whatever has the trigger group in it to be the "gun", so a T/C barrel is technically not a gun. That's why you can buy all the AK kits you want, but if you want the receiver, you either have to be a legit manufacturer, or go through an FFL to get one. The same goes for AR-15 clones, with the lower receiver being the actual point of interest.
I'm sure it's plenty legal, since DW has been selling interchangeable barrel guns for years, and I believe you can buy conversion cylinders for sixguns without being an FFL-holder. In that respect, those new cartridge conversion cylinders for blackpowder replica guns are apparently taking advantage of a "loophole". (Read: Remaining liberty.)
But who needs real reasons to ban this gun? The powers that be will just make something up, like, it's made of x-ray stealth plastic, or it shoots teflon bullets that only kill cops, or that it could be converted to 50 caliber and be used to shoot down jet airliners.
Sorry, El Hombre, but you mentioned the legal aspect and now I'm cynical. I bet when they do make it here, the barrels will be tack-welded on, for everyone's safety.