you can twist the spokes into each other to effectively shorten the length.
PITA to replace broken spokes, but wheels with spokes bent around each other are stronger than conventional wheels.

something like this
http://www.instructables.com/id/Twis...ng---flowers!/

if the wheel allows it, you can also go to a 4 cross. But these would be on 36h or 40h 700c wheels.