Ok, that's helpful info. After reviewing the bridge implementation I don't believe that we will end up needing this. It's a nice to have for now, but ultimately we don't want the custom code or transformation code. But for now if you want to provide both vertical and horizontal interfaces that's helpful. Also it looks like there's no need for any custom support for multisig.
I'll look into vertical storage. In that case we would not need to pass the extra key field, as it would just be an independent column we would retain for correlation.