We probably won't get a definitive answer or an editorial change until something like this happens in a high profile power 5 game. But, IMHO, the best thing to do is consider anybody in the line area (between NZ and backfield - yes, this includes "mugwumps" or "no man's land") as being on the line (regardless of his motion or where he may have come from), and consider anybody in the backfield area (not beyond waist of nearest lineman) as being a back (regardless of his motion or where he may have come from).
If such player is in the line area at the snap, treat him as a lineman, i.e., linemen inside of him are "covered", and he can be eligible/ineligible according to his location on the line and his number. If he is moving when the snap begins, you either have an illegal shift that converts to a false start (dead-ball), or illegal motion (live ball). If he covers someone, and that someone commits IDP or ITP, "bad on him."
If such player is in the backfield at the snap, then treat him as a back, i.e., he counts as a backfield player. If he is moving at the snap, then his motion is either legal (started from the backfield after a shift is completed), or illegal (moving forward or started from the line). If he is the fifth (or more) back, then you have ILF.
I wouldn't mind seeing an editorial change that not only clears this up (with the results I have described), but also declares such player illegally in motion to be ineligible (from the line or from the backfield).
Until then, if these things happen - get video.
Robert