California's AWB begins with "any centerfire rifle." That's right, evil features are a-ok on any rifle using a different priming method.
While working up an AR-type rifle in .22 WMR, .17 HMR, or even .41 Swiss certainly has potential, I wonder if somehow we can resurrect the pinfire. Use a two-part case that can be disassembled to remove the 'pin,' and a standard rifle primer mounted sideways that can be decapped an re-primed somehow. There'd be issues with fragility, and stacking and then aligning the pin with the chamber. I'm thinking a pin would be easier than a cylindrical case with a side primer, since there's no projection there to align and actually hitting the damn thing would be a crapshoot.
That, or someone could cook up a rimfire 5.56x45mm. Merely for malicious compliance purposes, of course.