From Jesse's comment on dev:
What I would like to propose is a simple interface that people can use to implement a 'constraint' (matching the classic database definition). This would help ease of adoption by helping HBase more easily check that box, help minimize code duplication across organizations, and lead to easier adoption.
Essentially, people would implement a 'Constraint' interface for checking keys before they are put into a table. Puts that are valid get written to the table, but if not people can will throw an exception that gets propagated back to the client explaining why the put was invalid.
Constraints would be set on a per-table basis and the user would be expected to ensure the jars containing the constraint are present on the machines serving that table.
Yes, people could roll their own mechanism for doing this via coprocessors each time, but this would make it easier to do so, so you only have to implement a very minimal interface and not worry about the specifics.