We are planning to use the Tx Relay API for a gasless user experience. Our fiat-onramp Monerium recently added EIP 2612 permit approvals to their stablecoins (EURe, GBPe). So they should be supported by the Tx Relay API now as well. However, because their token contracts are not upgradeable, the permit signature has to be submitted to their controller contract (see more details here https://github.com/monerium/smart-contracts?tab=readme-ov-file#erc2612-permit).