You guys have it pretty much spot on in terms of rules and regulations - keep it nice and simple to attract more people. I don't think it matters what year the bike is from as long as it meets the 1" shafted stem and caliper brake that doesn't work rule.
Mid-school was really the start of new school (e.g. the GT Fueller mentioned above had a 1.125" clamp-on stem and canti-lever (pre-V-brake) mounts, and a lot of those bikes would still be good by today's standards (e.g. Fueller, S&M Holmes & Dirt Bike, etc).