We can on the 747-400.... It is all to do with Vmcg and other complicated stuff that my knackered brain is trying to remember. It is strictly not a "reduced thrust using assumed temperature" but the thrust is reduced...
Roughly, I think, We can lift more off a contaminated runway (sometimes) using reduced power because in the engine failure case Vmcg is artificially higher... because of the reduced thrust.
It is a seriously complicated calculation with many variables of contamination type and depth, runway length and braking action, and one that is best avoided if at all possible.