My gut feeling is maybe a Rohloff tensioner might be a good upgrade from the plasticcy thing it comes with. Don't really know why Rohloff's need a tensioner but the one's I've seen are pretty well engineered...
http://www.sjscycles.co.uk/product-R...250)-12312.htm
In my own current Brompy/Merc thread I mused about fixed and pretty much everyone is saying it's impossible; this is because the hinge of the rear triangle and the centre of the cranks is not the same so that when you flip the triangle underneath the chain tension changes.