Has anyone tried using an overhead projector with an enhanced UV light source to projection print onto photo emulsion-coated screens? This would eliminate having to "tile" multiple inkjet transparencies to make large serigraphs.

Yes, that is actually done in commercial industrial screen printing. You will need a very fast emulsion. Some emulsions are made for projection exposures.

But of course they do not use overhead projectors. Rather they use projection exposure units and vacuum tables made for this purpose. So for you it will be all about that bulb and how much UV it can put out.