Wish I had seen this earlier. I do not open abscess capsules. Personally, I find it quite old school. Can't say I would never do it but in all the abscess cases I have treated, I have never had to open one. But there is one thing now that I think merits a discussion with your vet. Another old school treatment and that is the use of a Sulfanomide in a rabbit. Sulfa drugs like Septra, Bactrim, Sulfatrim, etc. are quite stressful on the beneficial bacteria in the GI. Most likely Floppy's gut is a bit stressed already so I would be cautious about putting a drug like sulfa in there that we know isn't very effective. I like Convenia and use it quite a bit in Penicillin resistant bacteria treatments. My preferred treatment for an abscess is Penicillin (the particular version I use isn't available in the UK but there is an alternative) along with Azithromycin. I would also urge more aggressive pain management. In addition to the Metacam, something like Buprenorphine or Tramadol would be quite effective.
If your vet would be interested in my abscess protocols, I would be happy to share in detail with him/her.
Randy