Having just done a long journey on a 125 with L-plates with stints on fast A roads, I don't see why there would be a problem. Fair enough the ET4 for example only topped out at around 55 to 60mph with a bit of a drop in speed in the uphills.
Besides I thought the aim of this was to raise a lot of cash for charity and get as many riders as possible to join in the fun. Shouldn't this ride accommodate all engine sizes? Experience of the riders shouldn't be an issue, as long as the more experienced riders watch out for us young'uns (so to speak) then those of us with less experience shouldn't be feeling out of our depth, plus there's the bonus of gaining more riding experience.