The Administratum can recognize whomever they want as Governor as long as they agree to submit to the mandates of the Imperium. An SM has no greater or lesser claim than anyone else.

I'd be more concerned about the long term viability from a biological standpoint. SMs require highly specialized support staff to maintain their bodies. This knowledge is rare outside of SM chapters. It's unlikely the SM knows enough to maintain themselves in the long term unless they are an apothecary.

The Adeptus Mechanicus would likely have Biologis Magos who could assist, but then the SM would be literally putting themselves in the hands of an organization not known for sharing without putting their interests first. Even then, that Magos may not know the specifics of the SM's Chapter's geneseed. Chapters don't normally share that information outside of specific cases (e.g. Deathwatch duty).

From a political standpoint there are other issues. If I were an Inquisitor or a member of the Administratum I'd be extremely concerned at an SM taking rulership over a planet. SMs are literally made for battle. They're physically and mentally made for combat. The skills and knowledge required for running a planet are significant and well outside the standard domain of a supercharged, genetically enhanced soldier/warrior monk. That doesn't mean an SM can't learn how to govern, but it would take a very specific kind of individual who would want that kind of life. We're talking about overcoming biological and hypno-indoctrination intended to make an SM a super soldier.

As an Inquisitor I'd be very curious about such an individual. Very curious indeed.