I have more experience with equine acupuncture; and I am a pretty big fan.
I had a 33 year old very beloved large pony who was stiff and arthritic, particularly in the stifle and hip. He was uncomfortable enough I was considering euthanasia so I gave acupunture a try. (There's a vet in the practice I use who's a licensed acupunture practitioner.) Sweet old pony stood like a rock and didn't move an eyelid while the needles went in. When they came out, he had this really funny look on his face; then picked up the lame leg and gave this enormous, head down, back arched stretch with his stifle snapping, crackling and popping, groaned and then *trotted off sound*.
It was amazing, and if we had thought to video it, acupunture treatment would be booming. I had him treated several more times with good, though less dramatic, results before I did have to put him down at 35. Acupunture gave him two more years of decent quality life.
I will say that I think in most cases it's palliative rather than therapeutic (pain-relieving rather than eliminating the cause.) If you're out of other treatment options, I would give it a try on your horse. The cost is not outrageous, and it's minimally invasive and very low risk.
As far as chiropractic, I am a great believer in it for people, a moderate believer for horses. I have known cases with good results, and some with very poor, where the horses were worse afterwards. I think it depends first off on the practitioner and second of all, on the root cause of the horse's lameness. Since xrays are inconclusive for your guy, if you find a chiropracter with good references and who does a lot of the stretching that boxer talks about above, it may also be worth a try.
Good luck, must be incredibly frustrating.