We have lots of green grass in the spring and summer and we have two horses that we ride and show and we don't want them getting overweight. Would putting them in a dry lot/round pen at night be a good solution?
Yes, green, growing grass can indeed be a big problem, especially when most of it is 'improved' cattle fattening varieties. I'd be putting them in a 'dry lot', *with free choice low NSC hay* for some period of each day, which will reduce their intake of the rich stuff without leaving them hungry.
See also thread by Saddle in health section 'esc. article on feeding the horse'.