A seperate suggester widget sounds very good. I looked at the current state of _buildPropertySelector and it doesn't look like it would be easy to override but an option would be good or the .entityselector( ... )  part could be extracted while all the .on and .prop calls would stay there.

The current implementations needs to inherit from entityselector as all cases where no property id is requested or no entity id is available are forwarded to the entityselector (when __useSuggester() == false).