A suggestion:
Looking at FAA documents, I can't cite a regulatory definition for "established" on course either. But I would suggest that in the situation described, waiting for the CDI to be nearly centered and the heading to be approximately aligned with the course before accelerating to a new speed would be prudent and in keeping with the presumed intent of the restriction. In other situations, a different standard for "established" may be appropriate.
This question often arises when discussing the situation of ATC providing a vector to final along with an altitude to maintain "until established". These discussions often end up examining the "why" behind the requirement too. As it should.