I've been reading about Barney for a while now and greatly appreciate what you are doing for him.
I have used Metacam on several of my cats and have found it useful, as have some of my rescue friends. It's used differently by different vets, with some believing it can be used only once. I noticed a huge difference when I used it on Ginger whose joints were getting very creaky since she's now 22-1/2. After the initial two high doses and a few days on the much lower dose, she actually started jumping onto my high bed again instead of struggling to get on the box and then scrambling up the bed cover. It was incredible, and wonderful to see her trotting around again and smacking the other cats. One of my friends used it on a rescue cat that had a really painful wound and she also noticed great results. Hopefully, your vet uses it the same way - two or three daily high doses followed by tiny daily doses (Ginger weighs about 5 pounds and gets .1ml daily). Your vet may not agree with the initial high doses, in which case you are going to have to wait quite a bit longer for it to make a real difference. Laura has just put Amy on it, but her vet didn't use the initial high dose either so I don't think she had seen any difference before she left for her boat.
It's quite a battle when they have several things wrong with them, isn't it, especially when you have to give them treatment that is obviously painful. I have to wrap my Diego twice a day to give him a strong pain medicine. The problem is that it is nerve pain and in his mouth, and the medicine can only be given orally. I have to brace myself twice a day by saying "Five minutes fight and pain and 11 hours and 55 minutes of enjoying life, we will cope."