OK....I am going be be quite blunt here both to help this rabbit and also to drive home some points across the board. No intention of stepping on anyone's toes...just shooting from the hip...so to speak.
In this particular case, I see from the profile that the location is in PA. Contact Penn State University as I believe they have a veterinary program. If driving distance is a concern, and would certainly be understandable in a long state like PA, ask their outreach program for a referral as they work all over the state. All medical schools (human and animal) have outreach programs that do reach into rural areas to assist both patients and clients in situations like this.
And if you have any vets that are worth their diploma, vets consult with each other all the time. I have vets consult with me all the time. I have had vets text me in the middle of surgery for information. Challenge your vet....dent that ego that they can't figure out something. Works wonders if the vet is any kind of doctor at all. I have no problem at all in consulting with a vet on small domestic mammals (I do most of them) or wildlife.
Now, if their is mucus in the fecal material....it ain't no virus....it's bacterial. I really think we need to put an end to the concerns about a virus in the gut. No doubt that studies have shown there might be viral agents present in the gut....but those same studies also show large numbers of pathenogenic bacteria.....Clostridium and sometimes E Coli.....regardless of viral presence. And if you have a large population of Clostridium or E Coli in any species, that particular individual has a problem. Even the few studies published also note that the bacterial overgrowth is the most pressing issue. And any first year medical student knows you can't really treat a virus....only offer support such as fluids.....but you can treat Clostridium or E Coli and if you don't, it has a way of resolving on it's own and it's never good.
I overcome this issue all the time in stressed domestics and in every wild cottontail weaning.  I hydrate by any means necessary to maintain hydration....oral, greens, sub-q, IV, IO....whatever it takes. I attack the Clostridium. And as I have mentioned, it is an obligate anaerobe. Metronidazole (Flagyl) is the preferred treatment. I also bind the toxins.....currently using BioSponge but have used Questran in the past. And honestly, it wouldn't hurt to drop a round of Fenbendazole (Panacur) since parasites are most likely present. And provide a stable and quiet recovery zone. And stick to a very basic rabbit diet. Steady the ship.
Time is limited in dealing with these issues. The longer this issue is allowed to play out, the more likely that gastric ulcers will form and those little lesions can become major players. I have run things out about a week but even I start getting concerned once we get out that far...and I am more concerned about such a young rabbit.
Randy