Back in the days before I switched to stronger opioids, I took codeine exclusively and later at some points it helped me a lot with withdrawals too.
It's not true what a lot of people say that there is hardly any difference above 300mg, 400mg, 450mg, or whatever dose people see as a ceiling. I noticed the decrease in strength is apparent between 450mg and 600mg p.o. With i.m. administration this changed and the actual ceiling dose when histamine release were overpowering opioid effects was at 900-1000mg for me, so I wouldn't see the point of going higher than 800mg.
As for the CYP2D6 blockade, I noticed that I needed an ~18 hours span between doses, so the effects were fully felt. Of course being physically addicted I re-dosed much more frequently, 12 hours were enough to get me high again, but if I injected 450mg i.m. at T +0:00, then injecting 450mg at T +12:00 clearly didn't cause as strong effects as the first 450mg dose did. Combining codeine with some enzyme blocker, e.g. fluconazole (I don't advise it, such antifungal drugs are very bad for liver), may both boost and prolong the effects (after I quit methadone, my immune system was attacked by candida, I used codeine to alleviate PAWS and received various antifungal drugs, fluconazole let me decrease codeine dose even down to 300mg i.m. in the morning + 150mg i.m. before sleep).
Given the fact re-dosing isn't as effective as in case of drugs not needed to be metabolised to give active compounds, I think opioids such as codeine and dihydrocodeine could be used for short-term maintenance treatment, they're much much easier to jump off than buprenorphine and methadone. In addition they're cheaper in production and adding naloxone to prevent injections would make sense unlike in Suboxone.