xc-steve, I don’t really know the Plym valley but I’m guessing that you’re going to pop up onto the moor around Burrator ? The trails there are probably best equipped to handle large numbers as they’re rocky rather than peaty. However, the main Princetown/Eyelsbarrow ridge is peaty and is busy narrow singletrack and Burrator is probably the busiest corner of Dartmoor. Worth bearing in mind also that it’s a long way to the start for anyone travelling from up country.
End of the day, if people want to organise a mass group ride, good luck, I hope everyone has a nice time. I’ve just tried to point out a few reasons why it’s not been done before.