For example our aircraft aren't defined as dispatched until take off power is set
Folks,
That is close enough to the FAR definition for government work, the MEL applies up to the point of commencement of takeoff. It definitely does not mean commencement of taxiing.
Sorry I can't give a reference, too late at night.
PS: Have a look through a B744 or B767 etc. MEL, at the king of things that can happen between commencement of taxi, and ask yourself if you would want to get airborne.