Because it is not a flight one doesn't even need a flight crew licence.
I somehow doubt that. Sorry, it is a long time ago I looked into the law, but when memory recalls correct, the relevant paragraphs talk of "operating an aircraft" requires a license and operating starts at Master Switch ON. I remember some other "conditional permits" mentioned in the insurance papers, like special permits for A/P to move the planes at the maintenance org, but don't remember the details. During all my career as an instructor, i.e. a student was never ever allowed to switch the master on without a written flight order or oral flight permission when the instructor was delayed but already at the airport (naming conventions differ a bit at the schools) to document the instructor is responsible for everything the student does. At the bigger airports this paper/permission was also checked at security to not let a student alone to the plane without. The discussion about pre-flight checks - on many planes you cannot do that without power - was one of the always predictable ones. I remember one of my bosses taxiing a plane to fuel up and get into a ramp check at the tap with a lot of trouble, as he left all papers in his car outside security as he only wanted to make the plane ready for somebody else. From that instance I strongly suspect there is no way to do that without a proper permit, be it pilots license or other.
To the taxiing in IMC, do not light up all lights, as you may blind yourself, but choose sufficient lighting to see. I treated taxiing in IMC as to be done under "IFR thinking", means I always requested startup clearance or at least gave notice on the fields frequency - nothing wrong with using the time to do ground operations, such as refueling, i.e. when you plan a long X/C and know in about half an hour the sun will shine.