> Is interpolation of custom attributes only supported via EL expressions or also via BV 1.0 parameter interpolation? If only EL is supported, there is some sort of inconsistency which requires ${foo} to print out custom attributes but {foo} for default attributes.

I see these parameters on the same level as _validatedValue_ which is an EL thing. I am not sure I see the inconsistency atm.

> Another question is should we allow to overwrite default attributes? It might be confusing to see other values in the message than in the original constraint. We might forbid that, OTOH it's the constraint author's responsibility to do the right thing.

I don't see a reason why we should forbid that. It is up to the author in this case. We might want to add a test for this though.