SRB2 had to make source code-level changes in the Doom source code (not sure if it's straight Doom or one of the forks like PrBoom they used as a base). And Ashura's changed from UT2004 to the UDK, presumably because the latter allows greater flexibility (as well as not having to rely on a commercial game).
In theory, yes, he might be able to simply change constants to do what he wants. In practice? Not so much. It all depends on what functionality the engine exposes (and, if the engine is available as source code, how feasible it'd be to hack it about to do what you want).