Filtering conditions on units in availability search

I need to filter units during availability search, using values from fields attached to single unit (i.e. "office" and other). I cannot find a way since is (or, at least, seems) impossible to filter units before they are passed to GetMatchingResults() method.

My proposal, in attached patch, is to permit other modules to alter conditions before units are loaded. In my specific case this is extremely useful since it avoids to check for availability for all units at once.

Could you review and commit the patch or tell me if there is a more conventional way to filter units (i've tried to go with constrains or modifying search index but without success)?

Thank you!