If the metacam dose isn't very high, maybe your vet could up it a little. I know with my old rabbit, anything less than 0.4-0.5mg/kg twice a day wasn't as effective at helping her.
There are other products that could be helpful. I know of others using glucosamine, msm, and tumeric supplements, and they said it seemed to help their rabbits. A member on here started giving adequan injections and said he noticed an immediate difference with his old bun. I tried adequan but my rabbits condition suddenly deteriorated so the adequan didn't help. But I think if I had tried it much earlier at the start of her arthritis symptoms, that it actually might have helped. I think cartrophen is the UK equivalent of adequan.
Medirabbit (arthritis treatments for rabbits)
http://www.disabledrabbits.com/arthritis.html