I almost thought this thread was a joke when I read it
But seriously:
I know that it is illegal for human players to use any sort of mechanical device that would give them an advantage; this would include a stepstool of any sort. But as far as I know, the rules say nothing about coaches using stepstools. If you really are worried about this, go ahead and put it in the FIRST QnA.
I think this is a non-issue, though--we have a short coach (sorry Mark

) and he'll just move to a side or stand between the drivers during the match.
JBot