Validation for SharePoint 2010 external lists / external content types

Where does one put validation for external content types? Let's say one field on my ECT represents an ema开发者_StackOverflow中文版il address that must be unique in our system. Where would I put the validation that checks to see that someone hasn't submitted an instance of the ECT with a duplicate email address? How do I bubble up validation errors so they're displayed in the SharePoint UI?

So far the only option I see is to create custom InfoPath forms for the forms that require custom validation. There is plenty of resources out there on how to customize default list forms in SP 2010.

It sure would be nice to have more options. Seems like there ought to be a special exception class we can throw from the service class (that handles the interaction with the back end system) that the UI would treat in a way a validation message is treated. I have tried LobBusinessErrorException but got a standard yellow error screen.