Shimano and sunTour (and possibly others?) made top-mount shifters - BUT the shifter bosses are of different size and/or threading for the two companies, IIRC.

There were some neat double cable stops that bolted directly to the topmount boss, but those are rarer than hen's teeth these days. I just did it my way with the 710. I put the topmount screw back in just to keep it clear of water/debris...) I may find a use for that mount someday...