We do have the litd UI for creating LNC sessions with certain perms or totally custom perms. You can also use that UX for creating custodial off-chain accounts. Both of those use cases are technically a UI for generating custom macaroons, but it's restricted to those use cases.
I don't think we've put much thought into building a fully fledged UX for baking any and all custom macaroons, but it sounds like that would be useful and we should put more thought into that. Appreciate the feedback! 🧡⚡️🏗