Dingo to answer your question , firstly if I wish to show an engine or whatever at a rally that is being held by a club other than mine and that club is affiliated with the NHMA this could present a problem . That being the insurance cover that I have may not be acceptable to the NHMA, there fore that club has every right to exclude me from the rally.
Secondly ,the NHMA has a set of guidlines that the affiliated clubs should adhere to ..........these may be at odds with the guidelines that my club has . A standard set of guide lines or rules is the best solution .
As a point of interest I belong to two clubs ,my insurance is through the non affiliated club ,the other club is affiliated and I have no insurance through them.