RELAX NG allows users to write ambiguous grammars. People sometimes says RELAX NG is not suitable for something (e.g., XQuery and/or data-binding) because that something cannot handle ambigous grammars. But I don't think this is a problem. Those applications that cannot deal with ambiguous grammars can simply restrict the input grammar to be unambiguous.
This tool would hopefully serve as a proof-of-concept that it is doable to test if a grammar is ambiguous or not.