I am not sure where the best place to put validation (using the Enterprise Library Validation Block) is? Should it be on the class or on the interface?
Things that may effect it
The way you are trying to use the Validation Block with DI, I dont think its a problem if you set the attributes at interface level. Also, I dont think it should create problems in the inheritance chain. However, I have mostly seen this block used at class level, with an intent to keep interfaces not over specify things. IMO i dont see a big threat in doing this.