Matt, I would argue in your favor given the information you've stated and the rules as posted. Going from a 20lb coolant bottle to a 2lb bottle, I could understand. (i.e. my F250 holds a couple gallons of coolant and is quite thick to hold system pressure.)
a non-pressurized expansion tank doesn't weigh much to begin with and are prone to rot. Good luck finding one in good shape for a 20 yr old car, and they're stupid expensive at the dealership- If you can even get them.
If the factory bottle held 2qts and your replacement held 2qts and was mounted in the same location (oh noes!! he moved 2LBS in his engine bay!), then I'd argue that your replacement is within the intent of the rules and IT philosophy, and contains no competitive advantage.