You could also try adding wood ash to their dust bathing spots. If you have a fire pit where you burn wood or maybe someone you know does, you can gather the ash from there. If you suspect they have mites (I don't know how well it works on SLM though) you can actually rub it directly on them. It will basically smother any of the pests in their feathers.
I think oil, or something that would stick around a bit longer, would be better for SLM, since the ash would probably get rubbed off their legs fairly quickly.
I agree that giving them baths with soap probably isn't a good idea. And I think that it is possible to be too clean. As already mentioned, they have oils that they need for their feather condition but also there are beneficial bacteria on their skin as well as in their intestinal tract. When you start washing all that away, and giving antibiotics and whatnot, it throws everything out of balance. A deep litter system which, if done correctly, will encourage good bacteria to reproduce.
Are you able to move the run at all? Or maybe set up a rotating pasture situation? If any kind of livestock is stuck in one area, even if it is a large area, their chances of parasites increase dramatically. Free ranging helps eliminate that issue but I understand it's not realistic for a lot of people. Pasture rotation is a close second...
As far as preventing bumblefoot, I think making sure there is nothing that will hurt their feet in the first place is a good start. I'm dealing with it now and it's tough because the previous owner of our house left nails everywhere and other metal stuff that was allowed to rust down to tiny pieces. But there are any number of things that could hurt their feet, roosts with splinters, nails, sharp rocks, etc. I have stopped obsessing over this a bit and over time the problem does seem to be improving. I was giving one of my girls surgery every couple weeks, soaking her feet and rewrapping bandages every couple days. Finally I just let it run it's course and it healed the rest of the way on it's own. I just had another girl with it who I just soaked her feet, removed the scab and the kernel came with it. Wrapped her feet for a couple days and now I'm just going to let it be for a couple more weeks and check on it. Sometimes doing less is better, as long as you can find a balance...
