that´s actualy the point ... he who expands faster in orbit "may" have an advantage in taking control of the orbital theater but again at the expense of getting less metal than the player who manages to fully colonise a planet from the surface ... and naval extractors already exist or are you asking for submersive extractors...
It makes sense, I lost many a game because I neglected orbit, after feeble attempts of fleeing for another planet were ended by a surprisingly large amount of orbital fighters, but my point is just how would this work? the orbital extractor I mean. Would you be able to build them anywhere?