I use flax seed, as well as whole sunflower seeds and oats, for Fey (and Sprite) who has trouble maintaining weight and had developed a rough double-thick coat and seemed unable to molt. This mix helped a lot with getting her to molt and her fur is now much nicer. I haven't noticed any improvement in her poops, which are often wonky due to her dwarf hotot genes/megacolon. I give her 1/4 teaspoon every other day. She has a sensitive tummy and I don't want to overload her with treats. The other buns all get it as an occassional treat. Oberon didn't tolerate flax seed at first and would get excess cecals but now he's more used to it. But because of that he only gets the same amount as Fey, even though he's over twice her size.
Oh and Stan, I bought the same brand of flax seeds! It was easy to find in the grocery store, in the baking aisle.
Edit: Rolled oats are sometimes given for mild diarrhea and they do seem to help.