The difficulty (in my opinion) of no pasture time is that one has to emmulate the natural trickle feeding regime of the horse. As you know the horse is designed to be on the move a good deal of the time, nibbling/grazing and browsing whilst roaming along.
To be out in a dry lot with a pile of hay or a haynet to eat out of will give some outside time, but little exercise. Some folk thinly spread the hay around the dry lot so that the horse moves aound whilst 'grazing' the hay, which works quite well to provide the natural 'movement while grazing' regime - any chance that you would be able to do something like this for Halo during the daytime? If she will be restricted in her outside time (not sure of the set up at the new place) will you be able to get over there daily to give her extra exercise?
You are right to watch the amount of hay that she will be given, but remember that you still have to try to get as near as possible to a 'trickle' feed regime for the safety and the proper working of her digestion - not so easy when hay/feed is given at teatime and then nothing more until breakfast the following morning! Perhaps you could invest in a slow feeder bag/net for her hay to help it last longer? I have never used one, nor even seen one, so cant advise you as to type, but I'm sure others will be able to help.
I am lucky enough to have the land available so my tribe live out all year round - just having ad lib hay put out in the worst of the weather - and I admit they are fat little piggies! LOL!! So when any of the girls are being bred from, they simply get a balancer - plus ad lib overnight hay - when they come in at night during the last month of their pregnancy, which I then continue to feed while they are in at night for the month after they foal - once babies are a month old and strong and healthy, they and their Mommas go back to living out 24/7 and the clycle continues for another year. The balancer I use is a Gro and Win one, which I think you can get over there in the USA.
By the way, I would love to see some updated pictures of your pretty girl .............. please.