Both Micro USB and MHL seem to be industry standards - ref Wikipedia. Lot of vendors seem to putting out cheaper Micro USB to MHL adapters - so even if (Micro USB->MHL) may not be a standard it is at least built on two standards and proprietary licensing / approval seems to be unnecessary for vendors.

Also most MHL adapters seem to do 1080p - the Lightning one seems to do 720p badly if we are to believe Apple store reviewers.