There was some discussion on the bitcointalk forums about using CLTV for
cross chain transfers.

Many altcoins don't support CLTV, so transfers to those coins cannot be
made secure.

I created a protocol.  It uses on cut and choose to allow commitments to
publish private keys, but it is clunky and not entirely secure.

I created a BIP draft for an opcode which would allow outputs to be locked
unless a private key was published that matches a given public key.

https://github.com/TierNolan/bips/blob/cpkv/bip-cprkv.mediawiki
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