I don't think it would really help much in your situation... which is a tough one.
This is just me, but if I was in your situation I would probably dose only 25% recommended dosage of both of those AT MOST. IMO it will help them because they will start scratching against rocks to remove the ich, sometimes to the point of injuring themselves. The melafix/pimafix will give a little help in recovery of those types of wounds, but will do very little for the actual ich outbreak.
There really are not a lot of options.. you can't QT, can't hypo, so your left with trying to let nature take its course, which is exactly what I do when I have small outbreaks... and I do about 2x a year. I can go months without any problems, not add anything new, and then for a week have a fish or two show small amounts of ich. I just make sure to soak all the food in garlic guard when I see that and make sure they are eating and acting healthy. In my case it has always worked... My potters angel and another fish started showing signs of ich about 2 months ago, about 5-10 spots total on the angel.. I put in a little medicine, removed carbon, made sure to feed with food soaked in garlic, and it went away in about a week. A month and a half now and I have not seen one spot on a single fish that looks like it could be ich.
The problem is, your case is much worse than mine was. Without other options like QT then unfortunately the reality is you may lose a few fish in the ordeal. I watched a fellow reefer go though that on his tank about 6 months ago.. almost every fish was 75% covered in ich, and he lost about 1/2 his fish from it, the others made it through and recovered.. all he used was Kick Ich to try and solve the problem though, and I have never seen that stuff work reliably or consistently.