Making a different restrictor plate should not be a big deal. A throttle stop is exactly the same thing, but is often a little to easy to adjust. Another alternative would be a rev limiter with a lower dropout point.
A smaller restrictor or a throttle stop will effect the entire powerband and a rev limiter only effects the top end. From what I have seen, the starts aren't the issue, it is the top end speed being beyond the control point of the tire to weight ratio at speeds where a slip can cause major damage.
Outside the box here, just brainstorming, but if you set a max post-race tire pressure, that could be a tool to slow them down. It will also slow them faster when they get off the gas vs. other options discussed. If you use nitrogen to inflate your tires, there is much less gain during a race.