Sorry for the late reply. It's possible of course but any newly-added command should be well-debated (in particular to avoid commands with dodgy or inconsistent behaviour and to not needlessly reserve more command letters which we are already running out of).
So the first thing I would like to be answered is: In which scenario would the precision of EEx / FEx, which is a 64th of a semitone, not be enough? I get that the range of those commands is limited (only values 1/64 through 15/64 can be used) and needs to be extended, but apart from that, I don't see much of a problem. 1/64th of a semitone is already well beyond the resolution of human hearing, and I think for the sake of consistency with other effect commands it would make sense to let them share the same resolution base.
In fact, I think only one new command would be required for the purpose of microtonal tuning, which would kinda work like the Set Finetune (S2x) command: The command would be centered around parameter 80h (no deviation) and values 00h...7Fh would decrease the finetune from that, 81h...FFh would increase it. The exact meaning needs to be agreed upon, it could be either 64th of a semitone like EEx/FEx, so allowing a deviation of up to +/-2 semitones, or it could be in 128th of a semitone to allow a deviation of up to +/-1 semitone, which may be more useful and would be consistent with your wish to have at least 1/100th of a semitone of precision.
It also needs to be defined how the new command works with and without custom tunings: Would it always be expressed in cents in the classic sense (with 1200 cents per octave), or should it rather depend on the distance between two notes of the custom tuning? Would that be useful or more distracting?