Implementing something like that on an emulator that emulates 3D hardware, can easily break games, and completely mess up timings. It's not the same as implementing a frameskip option to an emulator that emulates strictly 2D hardware.
NullDC and Demul both uses frameskip, you only can't decide to activate/deactivate it.
Demul in Interpreter and Dynarec and NullDC only in Dynarec mode.