My horses get very little grain and I think top dressing would not work out. Does anyone just put some in a bucket and leave out for free choice access?

I have done it that way but don't put  to much out at a time.   Wind can blow it away or it can get wet.

Yes I have done it that way. I put the tub close to water. Right now I am too dressing his hay pellets with it and he is eating them fine. If he leaves in his feed tub and eats just the pellets, he must be going back during the day and licking it up because it's gone by the next feeding. It took a little while for one of them to get used to them, but that horse is on grass pasture, too, so he may not need it. 

i use to put out a coffe can at a time and dress their feed with it  they would clean it up and then put more out