Well, there's a rub-on medication for arthritis in horses......I don't have the name of it on this computer.
It's Surpass and, yes it does work well. However, it's over $50 a tube and, since it's a topical, it only treats the areas where you apply it. For short-term use, it's great; however, for longterm use it might be cost-prohibitive.
As far as Previcoxx goes -- why not try Equioxx? It's the equine version of firocoxib. I used it on my 23 year-old TB gelding, in addition to front coffin joint injections, weekly Pentosan IM injections for a month, and Conquer HA gel. The combination of those four treatments made him much more comfortable, but I started them all at pretty much the same time, so it's difficult to pinpoint exactly what worked (or if it was just the combination of everything).
Although, I have to admit that Equioxx is not cheap, either -- it's around $8 or $9 per tube. I'm not sure if the Previcoxx pills would be any cheaper or how effective they would be. I did use Previcoxx for my dog when she was first diagnosed with osteosarcoma, but I don't recall how much it costs. Just remember, with any NSAID, you run the risk of ulcers and digestive upset.
Have you tried any of the injectables? I'm not talking about straight joint injections, but Adequan, Legend, and Pentosan. Those would probably be your best starting point.