Categories could be:
I think the cut-off should be 2000 because things began to really leap forwards after that.
Most old DH courses had big pedally sections in them, as well as rough DH bits, so the advantages/disadvantages of each different type of bike should balance themselves out to a certain extent. For example if someone was on something like DeliHustler's Super 8, then they'd fly down the rough bits but they'd be blowing out of their a**e on the pedally bits, whereas someone on a hardtail might benefit from the pedalling section(s).
You could even have an old skool trials section, so long as there was a small brook/river and some interesting looking sections of bank!
And don't forget the obligatory burger bar - or have someone on BBQ duty and ensure that one of the entry requirements for each person is a pack of burgers or sausages!
This sounds like a brilliant idea! All I'd need would be a bike