I am of no help. Just chiming in to say your post activated my rant mode. I drive the Merritt every day, and I’ve stopped counting the number of vehicles in tow I’ve seen on there lately, not to mention campers, trucks and commercial vehicles. The prohibited vehicles list is only listed at every entrance ramp. If it weren’t for the low bridges putting your horses at risk, I’d say just use the damn thing. Everybody else is. The police in Fairfield County obviously have zero interest in addressing these vehicles that are not supposed to be there. I don’t see their NY counterparts doing anything about it on the Hutch either.
As to your actual problem, I would think there has a be a truckers version of these Apps that would route you around the off limit roads. I use Total Traffic for my traffic management. It doesn’t plot a route for you, but it does show where the clogs are so you can make a plan B for getting places.