Putting a rabbit on a hay only diet for a couple of days WILL NOT KILL THEM or be harmful. Rabbits can and will get the nutrients they need from hay for a few days. It is not a recommended long term diet, but I have had my vet suggest it as well as many other knowledgeable members.
I have run out of pellets before and had to feed my buns a hay only diet for a couple of days until I was able to get to a place that sold my pellets. Non of my rabbits got sick or had any side affects besides being upset about not getting their daily pellets.
Putting a rabbit on a hay and water diet is the same as putting a dog on a rice and cooked chicken diet (granted, I don't know much about dog diets, and I don't even know if this is recommended for dogs or not anymore). It offers a bland diet that allows the body to work it self out. If it was caused by too much veg, then the hay only diet will allow the body to sort itself out.
If the problem is parasites, then a dewormer needs to be used, antibiotics will do nothing for parasites. Antibiotics are only appropriate for a bacterial infection.
I agree that if the problem is muciod enteritis, then more aggressive treatment is needed. But the symptoms of mucoid enteritis is much more severe than a few loose poops, it generally involves some mucous and large amounts watery diarrhea. However, the hay only diet will not make mucoid enteritis worse, but it will need more treatment than just a diet change.
I think that pellets should be offered plain, or with whatever it was mixed with before just with out the oats. Offer her the usual amount each day, and make sure they are fresh.
I would not take away the hay. She can, and will survive on hay. If she still refused to eat pellets, and you take her hay away, she could go into GI stasis from lack of food before she decides to eat her pellets, some bunnies are stubborn like that. Again, she will not die or starve on hay only. I think with the continued offering of plain pellets she should start eating them soon enough.
And if she doesn't within a week or so, then you can try a different brand or a fresh brand. And if she refuses all pellets, then you may need to figure out how to provide a pellet free diet (which is pretty hard to get all the proper nutrients a rabbit needs long term this way). First choice is to get her eating pellets again, as they are one of the best ways to provide the vitamins and minerals a rabbit needs for long term health.
-Dawn