I mentioned before that psoriasis is caused by auto-immune problems, so you didn't have to restate it.
As for marketing this as a remedy for sale, that is not my style. Plus, I don't know how well it would store if I tried to extract the juice and distribute it as lotion. Part of the joy of onions and the reason you don't want to put it on your face is that it gives off sulfuric acid fumes. This may have a lot to do with why it works. There is a chemical reaction going on with the juice as it is exposed to air, and the fumes aer part of it. I have no clue what other chemicals are involved.
Secondly, I don't know enough about how well it works. It seems to work fine on me, but you have heard of the placebo effect. It needs to be tested on other people, which is why I started this particular blog, to find other people willing to try it.
It is simple enough to try, just cut a slice of onion and rub it on one of your hands, and leave the other for a control. Or pick a spot on one leg. If your skin is really sensitive, maybe a very small spot, to make sure it doesn't hurt (like I said earlier, in an open wound it burns, but the wound heals very, very quickly).
If you find it works, and want to apply it to your face, cover your eyes with a damp cloth so the fumes can't get to them.