I once saw an ad for a tow bar with its own set of wheels, that was powered by a beefy cordless screwdriver. For a DA40, sounds like it's all you need.
Edited: Patent description here:
Portable aircraft moving device - US Patent 4915185 Description
And a manufacturer of this kind of stuff here:
Producer - Aircraft Power supplies - Aircraft powered TOW BARS - Aircraft TOW BAR
Last edited by BackPacker; 28th March 2012 at 11:14.