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.

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.