Sportives happen on public roads which are not closed for the occasion, so there's nothing to stop anyone riding the same course at the same time. It's a bit cheeky maybe, but as long as you don't try to take a free energy bar you wouldn't actually be doing anything 'wrong'. So how about a relay of riders in matching YACF jerseys on big old roadsters - all-rounder handlebars, rod brakes, balloon tyres? Get enough people in the 'team' and no one would have to ride far, so everyone could go flat out. Imagine the conversations among the entrants afterwards:
-Did you see that rider on the old Dutch bike?
-With a wicker basket on the front? They were going like the clappers!
-I saw him at the first feed station, he went straight past and didn't even stop!
-He? I thought it was a she. Never mind, I saw them near the finish and they were still flat out.
-Really impressive stamina.
-Yes. From the Yeovil Agricultural Cyclists Federation.
-Well, he did have a sheepdog in the basket.