B can sell the signature to their local channel partner S using payment points and scalars. A and S sign the settlement transaction, increasing the balance between them by amt + fee in favor of S and exchange the signatures. A sends the update transaction to B using an encrypted asynchronous communication channel. When B comes online, they decrypt the update transaction and sell the signature to S for amt. An issue is when the channel is forced onchain by either A or S. In this case, the UTXO should be claimable in both a secret-revelation path and a timeout path, both enforceable on-chain. This ensures that the funds are not held hostage by threatening unilateral close of the channel. Much of the advantage of this solution lies solely in S being directly a peer of the payer. It seems doable with just HTLCs and a normal BOLT11 invoice from B. Trampoline routing can then be used to mask B from S. There are also potential issues with privacy and locked up capital.