Not sure if this is technically possible with Grin, would be nice if it is. The only solution I can think of is something like sharing a users private key to a receive only wallet, which can only transfer to your main wallet, with other nodes on the network that can sign your transaction for a small fee. This receive only note should hold some minimal fee to avoid spamming the network. A, here we discussed it, think you sugested some of it such as the minimal holding value