If you are just referring to work that needs to be done on a vehicle when it breaks or regular maintenance, there is nothing in the HTA that says the work has to be done by a licenced mechanic. However, if the work were done improperly because the person who did it wasn't licenced, that could potentially open the company up to civil liability, since the company is required to do what is "reasonable" is the maintenance of its vehicles and a court could find that having work done by an unlicenced individual is not "reasonable". For a definitive answer on that you should consult a civil lawyer.
That being said, there are still some things that a commercial vehicle requires that only a mechanic can do.
For example: annual inspections. Also, don't forget about all the other commercial vehicle requirements: CVOR, pretrip, schedule 1, log books, etc.
NOTHING I SAY ON HERE IS LEGAL ADVICE.