Lock up ehash tokens in a P2PK output. The pool treats that pubkey as a bitcoin address and includes it in the coinbase of the block templates it broadcasts. When a block reward is found that ehash gets swapped for a Spillman channel. Ehash without this locking condition can be swapped for ecash.

Open question: can you ever unlock this ehash? I think no because that complicates block template construction. What if the user unlocks their P2PK ehash, sells it, and then a block is found on an old template that included that coinbase output? The user has effectively double spent those coins.

Simplest solution is to lock the coins with a long (infinite?) timeout. Once you swap your ehash for a coinbase output you can't unlock it again.