IANA blockchain expert, but this is my understanding:

You need to be able to show a "proof" to the bitcoin main network that the coins are being transferred back to the bitcoin blockchain. This proof needs to be hard to generate, which pretty much makes POW mandatory.

Do you know if that's 1) "must use POW for the transaction that burns the sidecoins", 2) "must have proof of enough total work in the sidechain atop the block in question", or 3) "must only have POW blocks in the sidechain"? Superficially, 2 seems most likely.