I think you are all looking at it a bit too biased like. You're all pro-gun and using common sense and logic, when we know those who enforce the laws and prosecute are rarely pro-gun, and almost never use common sense or logic. That being said I will point out two facts:
1. You have a receiver for a semi-automatic rifle with a pistol grip and a removable magazine that is marked for a centerfire cartdridge.
2. Once an assault weapon, always an assault weapon.
Do you really want to get into a 'no I didn't'/'yes you did' type of fight with a District Attourney? I know I wouldn't. Whether or not you are right it is just way too high risk, because all they would have to do is argue that at some point that upper would HAVE to come off for some reason like cleaning, repair, etc and then you have the same thing as someone who put a complete lower parts kit on their non-listed receivers. Even some gun owners would agree with that, and most people who know nothing about guns would just assume what they say as fact, and you KNOW they are going to screen the jury to get as many gun-know-nothings up there who will just believe what the DA says because you are an evil AW owning liar on trial.
If you REALLY want to do it, I would suggest either permanently attaching a piece of metal that cross the magwell on the top side of the receiver, or drilling and tapping through the walls and adding a screw and then grinding the head off. Then just take some hand tools to the mag (most of them are just hollow bodies and are easy to mod) until the mag slide up in the magwell and around the newly added piece of metal. The .22 mags work, but it would prevent any centerfire mags from being seated.
ETA: I just realized something. Like someone said, most 22 mags don't have that rear ridge on the back of the amgazine (where the follower's foot goes in the mag). The Vulcan uses the channel for that ridge to hold in their mags. *DING*! Remove the mag and replace the pin with a cotter pin from a hardware store for about $.05. AR mags can't be seated, but .22LR can and it would require no additional alterations to the receiver.