I'm sure you could leave the muzzle on during the day and take it off at night and then just decrease the time with the muzzle. Unfortunately I don't have much experience with this, as this is the first spring I have encountered this problem. But my horse was not to thrilled about going from being out 24/7 to being locked up half the day and was starting to cause some problems so that's why I'm ordering the muzzle. Your horses may not mind it as much as mine.
You might want to check out Katy Watts | Articles about Laminitis and sugar in grass
they have some really good articles about spring grass, pasture management, ect :)