The ciphertexts get generated when the deposit address is generated. So it already exists before he/she deposits BTC there.
If the BLS signature on a forked chain is published in OP_RETURN, they get slashed.
And even trying that is dangerous for them since only one honest node can reveal that to make them slashed.
The update is here.
It’s not fully inheriting the security assumption, but this is the practical approach that comes after this post.
https://ethresear.ch/t/octopus-contract-and-its-applications/