The best way to do it would be with smart contracts, and all full nodes as signers on the smart contracts, see my full scenario and description here, https://forum.wavesplatform.com/t/how-are-crypto-gateways-secured-and-what-would-happen-if-they-are-hacked/1860/6
What I’m suggesting is reducing the single point of failure at a gateways by introducing an ecosystem that allows multiple gateways to deposit and withdrawal the same proxy token. Or better the witnessing and transacting of proxy tokens could become a function of full nodes. This could be done using smart contracts. If user A sends Eth to a public address controlled by a multiparty (all full nodes) smart contract, once witnessed by X (2/3) nodes the proxy token is issued by the smart contract. Vise versa for withdrawals. Slashing conditions could apply for bad actors.