charles_m's assertion is true (so far as I'm aware) in Utah as well: the registered
weight is used in calculating registration fee/taxes, but the GVWR on the vehicle data plate
is the number that matters when license class is called into question.
Also.. at least in Utah.. it seems a person can own and register/license a vehicle they're not licensed to drive. Driver licensing applies to operation
of a vehicle, not to ownership.
As SweetBearCub has found, it's hard to bootstrap when a person wants to become licensed to drive but doesn't own a vehicle in the desired class. I certainly wouldn't throw any stones at a person who bought a vehicle they weren't yet licensed to drive and then used it to go do their road test... Just sayin'!
Don't forget that big trucks are not the only class A vehicles. A carefully-chosen late model pickup truck towing a trailer can easily be class A and might be more readily borrowed for the purpose of a driving test. There's some disagreement among the DMV, DOT, and driver license division people I spoke with in Utah.. but by my interpretation of our rules: for example, a 2012 Ford F-350 2-wheel drive dual rear wheel truck has GVWR 13300, and if one tows a Big Tex 14000 pound dump trailer, then the combination is 27300. It's into CDL territory because the combination is 26k or greater, and it's class A because of the trailer: class B CDL only allows trailers below 10k. Of course the personal/non-commercial use exemptions which have already been fiercely debated also apply so individuals towing their horse trailer, toy hauler RV, or whatever don't actually require CDL licensing. Contractors moving a trailer full of concrete forms however do require CDL class A and do routinely get ticketed if they're found with less.