[iso8859-1]

Bitcoin to Stacks is supposed to be the easy part...

The question was about how you move Stacks to Bitcoin.

I take from this handwaving answer that it isn't actually possible.

Or let's rephrase the question in Bitcoin terms: While the BTC are locked up on the Bitcoin blockchain, which key is necessary to unlock them? Surely one controlled by Blockstack, no?

[jude-]

You'd need a STX-collateralized relay service that would trade your xBTC for real BTC. The collateral would be burnt over time if the service misbehaved, such as by not doing outstanding trades in a timely manner. The collateral itself would live in a Clarity contact so it could validate proofs that the service sent BTC to xBTC sellers.