Traditionally, they use ullage motors, right? (http://en.wikipedia.org/wiki/Ullage_motor).

Perhaps they could dock with it a little harder than is strictly necessary or use a maneuvering thruster burn.

It seems like it'd be easier to use non-cryogenic fuels (hypergolics, or kerosene + LO2) which may make the piston idea workable. How do they keep cryogenic fuels cool in space in the long term? good insulation and vent a little of it off regularly?