I think a metal chassis would potentially have a big advantage in keeping the weight of the car as low as possible and give the car an advantage with added stability. I might be missing something here about the availability and ease of use of a metal chassis but I would be concerned there would only be a few members that would want to put the work into finding the right one and learning how to set it up.
I am worried about making the rules too difficult for members to participate. It’s already challenging for some members to prepare a spec car much less a builders car. I think it would add another layer of difficulty that might push members away from running the series. One of the key factors to consider in running any series is the level of participation. If only half the club participates then it’s not a great series to run. The easier we can make things for members the better it will be for the club as a whole.