There are a few fixed Twenties on www.fixedgeargallery.com - Maybe you could ask their owners how they did it - best way to find them in the mass of full sized bikes is do a marque search for 'Raleigh'
Cannondale Bad Boy / Mercian track / BOB trailer / Moulton recumbent project
Get out of the saddle and stomp it, same as any other fixie! The back end is a bit fishtaily and weird, but it does the job.
A fixie is more mechanically efficient 'cos there are no guts rolling around inside the hub gear: those guts have play and friction. The difference is about 12%, IIRC. Add fixie mojo and a fixed-wheel bike usually dances up hills.