Sounds similar to NIP 26 (https://github.com/nostr-protocol/nips/blob/master/26.md), but sort of the other way around. This could be useful for identifying/advertising the bridge, but doesn't solve the problem of the custodian being able to make the original user say things they didn't. Use of this tag would be optional too, so custodians could choose not to support it and be sneaky. But I know that's not the problem you're trying to solve.