I've read about a few breeding stations in Europe recently that keep stallions in a herd. One of them is the Swiss National Breeding Centre, which has successfully had two experimental stallion herds (one of 5 and one of 8 stallions together) since 2008 (the 5 stallion herd) and 2010 (the eight stallion herd). Mind you, these are adult breeding stallions, not youngsters. Not sure if they're being used for live coverage or just AI though.
Of course, precautions need to be made, the pasture needs to be big enough (they had ~12 acres for 8 stallions), there cannot be other horse traffic close to the pasture, and appropriate fencing needs to be in place.
But other than that, it seems to be do-able, seems to be a lot nicer for the horses, and had shown to greatly improve sperm quality, too.
(not sure if this video links properly, it's to a facebook page)