Hashrate Escrows (Consensus layer).

Would let coins move to separate side networks and back, with miners approving each withdrawal slowly over months.

This is the 'drivechain' design. Coins sent to a sidechain, a separate network with its own rules, are locked on Bitcoin. To bring them back, miners vote on a withdrawal over a long period, instead of a fixed group of key holders signing it. People who ignore sidechains are unaffected.