Actually the original DF-21 is a standard MRBM- the DF-21D is the Anti-Shipping version. The Chinese have indeed helped prop up the DPRK both in military and civilian material as well as various and sundry other things like food and cheap electronics. I would say the DF-21 may have been on the DPRK's 'Must Buy' list but the Chinese would not have wanted, in the post Mao era at least, to sell them such a weapon- especially considering it took an awful long time for the Chinese to develop and then deploy it. (Also, just while I'm saying this, it is interesting to note that Kim Jong-Il era rhetoric in North Korea was often quite anti-Chinese- mostly emphasizing their turn away from a purely Communist economic model to a capatalist one- the Korean ideaology of Jucheism was used in some ways to critize the path the Chinese were/are walking. Relations between the two countries are cordial, but their ties are firmly rooted in a past that China is rapidly moving away from and the relations are not as cozy as they were- China fears that if the DPRK collapsed hundreds of thousands, perhaps millions, of Koreans would cross the border with China to the ethnically Korean areas of Sourthern China, upsetting further an already economically quite poor region. Thus, propping up the DPRK, for China, is perhaps one way of making the best of a very difficult and un-certain situation.)
Still, putting a DF-21 in DPRK colors might make a nice 'what if' piece.