Currently the user needs to add constant meta data to the mappings and then build suitable queries using that additional meta data.
Native Compass support for polymorphic queries would be better for users (less user code, of course!) and means Compass is "truer" in its aim of being an Object to Search Engine Mapper.
CompassQueryBuilder now also has a CompassQueryBuilder#polyAlias(String aliasValue) which will return all the aliases/classes matching the given alias or extending it.
Can you take it for a spin and see if it match what you were thinking about?