Let's re-phrase this.
We know that it's an AR-15 lower that is the restricted part. I have to keep a zip tie wrapped around one of mine, even though it doesn't have a trigger group, grip or stock. Heck, I need an STATT to take it to a gun smith, even though the worst I could do with it right now is club someone in the head with it.
We know that an AR-compatible upper can be bought without any registration/transfer issues. Furthermore, the same is the case in USA - you can basically buy the upper without going through an FFL dealer.
So basically the upper is just a gun part, similar in classification to a magazine, while the lower is the firearm.
Now, we have a law on books that is interpreted as saying that magazines intended for use in a pistol are OK for use in a rifle (Case in point, Ruger PC9 and Beretta CX4 Storm carbines that both take pistol mags).
At the same time we have another precedent - SIG 552 lower (that uses STANAG mags) can be attached to the Swiss Arms upper (with minor modifications). SIG 552 is considered prohibited, in case of both rifles the firearm part is the upper, so the conversion is not only legal but is in fact desirable - you can use LAR-15 pistol mags with your SA upper.
So non-registered parts of the firearms can be interchanged with impunity (with exception of barrels, where barrel shorter then 470 mm might result in change of classification from non-restricted to restricted).
So the question is....
If one were to design a lower that is sufficiently (I don't want to say 'functionally', since the function of this lower will be the same - to feed the ammo onto the chamber, and to hit the firing pin at the pull of a trigger) different from AR-15 lower - say without pistol grip and with a single piece wooden stock attached directly to the lower (without the visible buffer tube), with the trigger assembly being different from any existing AR-15 trigger groups (again, say, borrowing heavily on SKS trigger group, and heck, with the mechanical "block the trigger" safety instead of a sear disconnect), would the resulting contraption be considered an AR-15 variant based on the virtue of the fact that it can accept AR-15 compatible uppers? This hypothetical lower would look different from AR-15 lower, would have a different non-interchangeable trigger group, would have a different visual appearance, etc then AR-15 lower. (Yes, it's not simple to design, since upper imposes a number of constraints, but this is a hypothetical question)
If it would be considered a variant, then on what grounds? To me such situation would imply that each specific firearm would have to be individually classified as restricted or not based on the length of barrel of the upper attached. Lots of make work for CFC! They could keep Canadians safe by verifying what upper an owner put on the gun today. (In fact that's probably one of the reasons why AR-15 is restricted - too hard to keep track of what length upper owner attached to the lower. Yes, I know that RCMP would rather see it as prohib, but realistically, that's a lot of burden on CFC if AR-15 was mixed classification based on the length of the upper).
In fact, I can think of a kind of a relevant situation: Wolverine Supplies have mentioned in an XCR thread that they will not be importing XCR barrel/bolt kits in lengths other then 18.6" specifically so that there are no classification issues with the CFC, even though in USA Rob Arms makes shorter barrels available. So this is a case where a part is specifically kept out of the country so as to not mess up classification, even though part exists and is available for sale (and presumably for import if one were to jump through enough hoops).
