“VMC must be determined with the most unfavorable weight.”
How, practically, does the FAA determine and load the “most unfavorable weight” for airplane it tests for Vmc?
"Weight and C.G
For rudder limited airplanes with constant aft c.g. limits, the critical loading for VMC testing is most aft c.g. and minimum weight.
Aft c.g. provides the shortest moment arm relative to the rudder thus the least restoring moments with regard to maintaining directional control.
VMC should be determined at the most adverse weight.
Minimum practical test weight is usually the most critical because the beneficial effect of banking into the operating engine is minimized. Light weight is also desirable for VMC testing because the stall speed is reduced."
Do they use midget pilots, remote controls, partial fuel, or ghosts?
Loading will be whatever is reasonably achievable. If the weight is determined to be limiting, there will be a minimum weight declared, usually in the permissible CG envelope. Vmc determination is not a straightforward exercise and usually involves a progressive workup to establish a static Vmc (rather akin to the usual pilot endorsement demonstration exercise) with subsequent dynamic Vmc checks.