PLEASE do not lunge and at this stage of the game PLEASE do not canter!!
Have you a round pen??
If so you can use it to diet and exercise!!
Just gentle jogging to start, nothing strenuous, you could do real harm.
As Magic said if she is still quiet to drive you could take her out for a couple of short, gentle drives a day- that will do it, too.
Horses eat less at night so if it is safe to do so I would turn her out at night and in during the day.
Grazing muzzles....I have not found they work that well on Minis, not that I have ever used one myself, I really do not like muzzles at all, but if that is all the pasture you have it wold be better than her not going out.
I would also have the Vet pull blood from her just to make sure there is no other reason for her being overweight, and I do not necessarily mean pregnant (
) If you do get the Vet involved you can get him/her to earn their fee by making you up a diet plan, too.
Hotwire is the horse dieters friend
You could section off the pasture and strip graze it- it does the grass no end of good as horses are picky eaters by choice, and to be grazed down all over benefits it.