The atomic swap as I see it would have plugins for each blockchain to support. If a witness wanted to support HIVE->ETH swaps then they could opt to only run that plugin if they wanted.

But each plugin would work together with the related blockchain to verify the transfer/balance. So I think for ETH, the wHIVE (wrapped HIVE) contract would work in conjunction with the Hive atomic swap plugin.

I mentioned two ideas in the post, one having the swaps coded into the blockchain either via a plugin or something else, and the other is enabling swaps thru the use of Oracles. I far prefer the former option personally.