Ledger device as an ephemeral signer

[joostjager]

Described in #153 is a use case where covenants are simulated by using a presigned 2-of-2 multisig transaction where one of the signers uses an ephemeral (random) key that is immediately discarded after signing.

Would it be possible at all to use a ledger device as an ephemeral signer?

Note that there would be a two-step process where first a key is generated and the public part of it returned to the host system. Using that ephemeral key, the host system prepares the "funding" transaction. As a second step, the host system offers a psbt that spends from the funding transaction to the device. The device recognizes the ephemeral public key, signs with it and safely removes all key material from memory.