When I went on Subs I had to have all other opiates out of my system for 48 hours. The sub dr is going to drug test you (should) and hopefully will help you ween down so that the 48 hrs is as doable as possible. DO NOT take a sub with opiate based meds in your system as it will bring on percipitated withdrawal which is worse!
I can't speak to opiates at all. My history as been with alcohol, benzos, and barbiturates. What I found is that there are indeed some places that can make tapering from those much more manageable, and that's what I did when I recently went off benzodiazepines. I'm not sure if the same is true for opiates, but if it is, in-patient is the route I'd go if I ever needed to do it again (my drugs of choice or otherwise).
Okay, so the thing I'm concerned about here ... what's in place to manage your pain? Do you take other meds like gabapentin or something for the neuropathy? If there's real underlying pain, I am also concerned for you in that regard, especially with neuropathic pain. It seems unfortunate that a pain clinic kicks a person out over one test. I think if it were me and I had real underlying neuropathic pain, I'd probably argue against it, even if it were to no avail, and I'd probably try to be open to alternatives outside opiates if I were not already on them, although I know there aren't many.
Hoping the best for your transition, and also for your pain management. Please check back in with how you're doing! I only visit here every week or so, but would like to know what you end up doing for the pain and how that plays out.