If we assume that the pain killing features of opioids against mental and physical pain share the same mechanism (!= euphoria) and your origin of pain isn't something that increases steadily like a tumor then nmda antags might at least heavily extend that period of time. I know too little about the mechanisms of physical pain, but they seem to be able to prevent or even reverse pain memory so a combo with something anti-inflammatory sounds promising to me.
Nmda antags don't come without a price, I'm completely researching about their long term effects in regard to receptor upregulation but as the dosages required against pain are very low, way below what K addicts use for example, things don't look that bad. The most side effects will be initial, pain relief is instant but they can induce confusion, motor impairment etc. which fade away after some time.
Another thing you might already have read about is ultra low dose naltrexone, in the range of maybe 1mg. There are some good reports and publications about and it doesn't interfere with opioids like higher doses do.
at least when they are European or the same manufacturing (200mg dark red, 60mg dark green, "MS ID" written on them - possibly exist different brandings, what is relevant is that they are capsules and contain little waxy balls of different sizes. They are pretty soft and can be crushed e.g. with a credit card).
