I think it shouldn't happen. I agree that you are inviting overloading of the circuit which is a violation and hazard. If any piece of equipment with a 30 plug is rated over 16 amps 210.23(A)(1) will be violated.
The difference between 10-15amp recepts on a 15 amp circuit and a 30amp recept on a 20 amp circuit is that the likelyhood of the individual piece of equipment being plugged into the 30 amp recept exceeding the circuit rating is greater than the likelyhood that someone will plug in enough stuff to overload the circuit of the 10-15 amps recepts. If the odds were high that 10-15 amp recepts on a 15 amp circuit were overloaded the code wouldn't allow it. Of course in a non-residential setting you wouldn't put 10-15 amp recepts for specific loads on a single circuit if the loads would exceed the rating.
