Gene - as you probably know, that's an over-simplification since your pivot point is sprung and there are yet more factors involved in the manufacturer's recommendation, but it's good enough as a rule of thumb. So going your way:
WB=170". Axle to Bumper=63.5". Say bumper to ball=9.5". Added length to scooter=7".
500x(170+63.5+9.5)/170= 715# on rear axle rated.
(385+50)x(170+63.5+9.5+7)/170= 640# on rear axle proposed.
By your approach, and assuming the receiver itself is capable, I would be OK.
But - part of the reason I'm posting this is to get real world experiences rather than opinions.