How about three "tally bots" to tally the votes for the txns and sign for the account?
Basically a "consensus algorithm" in miniature; until the three bots agree, they don't sign.
- The three bots would create a private key for each Account they sign for and provide the signing hash.
- An Account has its Voting Assets and thresholds marked on its Managed Data.
It would be best if the bots received txns already authorized by the "Holding Account" (can someone explain how this might be done?) so it doesn't have to compute "Account Approval" on its own (though it can do it if needed).
Lastly, as part of the submitted txn each bot generates a new signing key for the next txn. It appends operations to the txn to add its new key and remove its existing key to the proposed txn.
Once all three bots have appended the operations to change to the signing keys along on the signed txn; the txn gets submitted to the system.