I wouldn't have thought that pasteurella would cause that kind of head extension either. It's a bacteria and doesn't always necessarily cause snuffles, but when it causes an ear infection, usually it seems to be more of a head tilt that occurs.
I came across this info on opisthotonus in rabbits. It's arching or hyperextension of the back and head. There were a few causes of it in rabbits, that were suggested. One was called nematodiasis, which is caused by the roundworm from skunks or racoons. The other was caused by a vitamin A deficiency. I'm not sure how possible or common the nematodiasis is. I couldn't really find a lot of info on it in rabbits, just that it could happen. What I read suggested that it may be treatable with the bendazole antiparasitics. There was a little bit more info on the vitamin A deficiency, especially in it happening in young rabbits. It seemed that it would happen because of the vitamin breakdown in poor quality or old feed, so the rabbit or momma rabbit wouldn't get enough vit. A and so wouldn't pass it on to her babies. There was mention of it causing hydrocephalus and the hyperextension of the neck. i'm not sure on the treatment for these either, since there isn't a ton of info on these things. It would seem like with the vit. A deficiency, that getting a better quality fresher feed would be important, and that fresh leafy greens would help. There are also small animal vitamin drops that could be used. I don't know if you feed your rabbits veggies everyday, but that might be something you want to try, if you don't.
There is also the possibility of e. cuniculi being the cause, though I haven't ever read or heard of it causing the head extension like that. E. cuniculi is also treated with the bendazole meds. The treatment is fenbendazole at 20mg/kg, once a day, for 4 weeks.
With the nematodiasis, some places I read that there was no treatment, and others that oxibendazole may help, but I'm wondering if fenbendazole would be equally effective.
http://veterinarymedicine.dvm360.com/vetmed/article/articleDetail.jsp?id=385181&sk=&date=&pageID=5
http://www.bio.miami.edu/hare/tilt.html
Maybe you could find a different vet that you could talk to about these different possibilities with. It might be good to print out the info and bring it with you for the vet to read. The vit. A would be the easiest to treat, then e. cuniculi. The nematodiasis seems the hardest. But I hope all this info helps and that you can find a good vet that will help you figure out what is going on and how to fix it.