From A Biographical Dictionary of Later Han to the Three Kingdoms (Rafe de Crespigny):

In 199 Dong Cheng was made General of Chariots and Cavalry, but about this time Emperor Xian asked him to organise a conspiracy against Cao Cao. Early in 200 the plot was discovered and Dong Cheng and his family were destroyed. -HHS 72/62:2338-43, SGZJJ Shu 2:8a, deC 96:268-271

So it would seem the answer is yes, that plot really did happen (though maybe not with any actual secret edict written on the lining of a girdle) going by the Houhanshu and/or Sanguozhi zhu.