It might even be possible to use an aluminium seat post shim (assuming the one you have is plastic) and continue riding like that. Steveroot who used to be the kingpin on the Dahon forums has ridden with worse cracks no problem.
AFAIK he rode one case where the crack was all the way through and around (so completely broken off) but he reinforced it with some sort of clamping arrangement and rode just fine.