Ethereum

What if USDC was also an appchain rollup sequenced by Circle?

It could still be used as a normal ERC20 on the respective chains' contracts (e.g. for defi), but at any time I could "lock" my balance and that would effectively bridge it into the rollup.

We're trusting Circle to custody and not freeze the RWA anyway, so the "inherited security" of L1 for rollup transfers could be minimal and thus very inexpensive, it could even be entirely offchain (though I suspect compliance would make it easier to keep it somewhat onchain).

Bridging back onchain would be just a matter of "unlocking" the balance on the desired chain later (which Circle's sequencer would validate and mint/destroy accordingly).

Lock on Solana, unlock on Base.