You are missing a point of the multi-caliber receivers: The letter I posted says that it would be exempt if it can fire ONLY rimfire ammunition, where as a Multi-caliber receiver is clearly made to fire more than just one type. You can argue that it means that it can fire multi-caliber rimfire, but try arguing that to the courts, or the 12 people who you have been made out as a criminal woning illegal weapons in a anti-gun state.
You can build it all you want, but I would advise against it. You HAVE to assemble an AW just to build the lower itself, so that right there tells you it's a no-no. Once an AW, always an AW, no mater if you change it so it's no longer an AW.
If you are really set on getting it made, at least consult an attorney first or at the very least ask someone versed in the laws, like Bill Weise.