I know Carrie at crunchybaby will let you send her prefolds to tie dye. She has an absolutly fabulous butterfly tie dye that is imho the best tie dye anywhere. it's stunning. she may, however be on maternity leave.
eta: I bet any wham that dyes prefolds would let you send her your hemp prefolds to dye though...though maybe they wouldn't come out as vibrant since they are unbleached?