OK...
We called the vet and had her call in a prescription to a pharmacy near us...with the mucous it was pretty obvious it was mucoid enteropathy...we used sulphamethoxazole/trimethoprimalso calledNovo-Trimel.
(There are some who don't like Novo-Trimel because it isn't as effective as some antibiotics...but following the school of thought that the caecum impaction may be caused by the bunn being stressed to the point that the newly formed flora of the caecum(still developing in young bunns)are disrupted...we didn't want to use a stronger anti-biotic and "nuke" what good intestinal flora he might have)
This was our schedule for about 4 days: Guffy was about 8 weeks old.
Every hour:
-Ovol 1ml for the gas
-as much pedialyte or water he would take
-put Guffy on a heating pad and massage the sides of his tummy...he was so swollen it was too painful to rub his actual tummy.
-after about 10 mminutes of heat...onto the floor to move around for 5 minutes or so then back in his cage (we had a heating pad tucked under his blanket and that was where he would lay down)
Every 8 hours:
-1/4 of a 5mg tablet of cisapride soaked in pineapple juice (or the breeder suggested a laxative called Agarol too)(but this was with a bunn with constipative ME.)
Every 12 hours:
-1ml of Novo-Trimel
-dose of Metacam
Because he was hurting eventually his water and food intake stopped...he would nibble here and there but it wasn't enough...he was starting to look punky (not lethargic...but def tired)(didn't help we were waking him up every hour on the hour!)our vet advised to stop pushing fiber because if he wasn't eating or drinking properly all more fiber would do is worsen the blockage...so we switched to unsweetened applesauce and baby rice cereal to get his blood sugar up...I know some people have said pumpkin works too...but when we switched from pumpkin to the apple mixture there was a definate immediate improvment!
It was just a waiting game after that...cleaning his bum and making sure he wasn't in any pain...it was a very long few days...but ME is brutal and even if you treat it right away it is usually fatal.

I've done alot of research since our boys got sick...all you can do is treat the symptoms, be vigilant and cross your fingers.
Once the worst had passed hubby and I scrubbed the whole bunny nursery...we used a 50/50 water and bleach solution on anything the bunns had come in contact with and then steamcleaned the floors and used a power steamer on top of that just to be sure. Since the breeders other bunns didn't get sick and it was just ours...we threw out all out hay and pellets and started with fresh stuff on the off chance that mycotoxins might have been the culprit...
I hope you can intervene in time...but it isn't your fault either way.
I have had rabbits for a long time...we have 10 currently and I was blown away by the speed of this sickness...it really caught us off guard and there was nothing we could do to save our boy Finnegan he was gone in 12 hours from the first sign of mucous...and he was less ill than Guffy! Guffy had the swelling and constipation etc...
Good Luck!
Danielle