Ron, it would be great if the Classic 750 and 500 could be run as separate races, but with the current 52 entries covering both those classes I doubt it would be viable. To be fair to the MMCC I think what they have done is the best way forward - run the two concurrently and if it is a success and the numbers grow then maybe they can look at additional races. But with a limit of around 110 riders they have a way to go yet!
I regret to say I can see it going the other way with numbers dwindling to a single combined classic event - which could be risky if fast 750s at the front come round to lap slow 250s and 350s starting later. And of course, as you quite rightly say riders who have a 500 and 350/250 would have to choose which to ride, further reducing numbers (and the attractiveness of the event to enter with only a single ride being possible).