When you are taking a driving test to obtain a licence, you are dealing with the government in order to get something from them. Let's forget the "right" versus "privilege" and "they are there to serve me" debates for a moment.
You are there to be examined by an examiner - you want something from them. The examiner has the power of government behind him/her (if not for real, then most definitely in his/her mind) and if you are aiming at a full-blown commercial driver's licence, the examiner may see it as his duty to protect the innocent public from anyone who may be a risk on the road.
I digress.
Fact is, the less "different" you are, the more you can blend in, the less you attract any particular attention or cause concern, the smoother those transactions tend to be. Remember the experiences of getting tags and registration for oddball ex-military vehicles that can get hung up on the question "which colour is it?" ( "green, brown, black") which will not fit into the box on the form that allows for one (1) colour?
You get the drift. You may have oodles of fun taking a 931 for the driving test, given the right examiner with an open mindset that finds that MV interesting and cool - or you will be in a hot place with devils and pitchforks, when the examiner is sceptical and suspects that you try to somehow abuse the process, in a way that he may not be able to grasp but has a strong gut feeling about. Maybe you attempt to obtain your licence somehow more easily than if you were to use a regular OTR tractor and, by the way, what do you mean your rig does not have to conform to the nitty bitty DOT requirements the examiner is oh so used to torture candidates with on the regular OTR trucks he sees all the time? Half of the petty stuff he is looking for during the pre-trip may not even be on your MV. What do you mean "that is OK"??? He'll SHOW you how "OK" that is, in just under a minute.
You get the drift. It sure is a nice idea and it may even work, but it is one of those transactions where the cost and risk may outbalance the fun really quickly.