On my Legacy, there's no device that holds the mainsheet from sliding to leeward when tacking. I've sailed on someone elses Eclipse, and I think he had a small traveler adjustment, but I think his mainsheet setup was non standard anyway, so I doubt it's representative of them all.
Sometimes when I'm trying to get close to the wind in mild conditions, I often pull the slider to the center or windward with my hand. It helps a bit, but I can only do this for a couple minutes at a time. Rather than getting upset I try to remember that the Legacy was made to be an inexpensive boat, hence the lack of a true traveler, and true foils on the rudder and centerboard, etc.
I've thought about trying to rig some kind of slider clamp, as some others here have thought about, but just haven't gotten around to engineering it. I just try to enjoy the sail.