MIPS (you seem to know this, but many might not) is an internal layer in the helmet, usually a very thin rigid plastic layer, that has some elastic nubbins/supports between it and the outer helmet styrofoam shell. This means that the inner shell is free to stay put while the outer styrofoam shell rotates.

This would be different than a cap in that the elastic nubbins should dissipate some of the energy. All this is a bit speculative, but that's what I'd go with.