Given the nature of trademarks, DC is pretty much forced to deny such a request since each request granted represents a dilution of the trademark and introduces a small risk of the trademark being lost to DC.
On the other hand it is not clear why their permission is required at all in this case. Trademarks protect against someone other than the trademark owner selling products displaying that mark, but, there is no reason to believe that the intention is to create this statue and then try to sell it to someone as a genuine Superman product. There therefore cannot exist any confusion in the market as to the provenance of the statue since the statue isn't in the market in the first place, and so it doesn't constitute a violation of anyone's trademark.
This would only be an obvious trademark issue if they also intended to create merchandise based upon the statue and then sell that, I don't know if this is the case since as any good slashdotter I never cared to RTFA.
Had this been a copyright issue things would be different.
(I am not a lawyer and these are the random babblings of an amateur.)