I think we are talking here about two different things here:
a) keeping salt permanently in a tank with catfish (not recommended as it will damage their kidneys)
B) using it medicinally as a last desperate resort
The first option is obviously not a good thing, in a sense the second isn't either, but in a desperate case like this, it may be better than doing nothing.
If you think about it, a lot of the medicines we get from the doctors are not particularly good for us either, in fact they may damage our bodies. I have a friend who's just done chemotherapy, no doubt that has done a lot of damage to her body, but it was a choice between that and letting her cancer spread.
In the case of your pleco- well, he's going to die if you don't do anything, so thb I'd take the risk.
If you post in the catfish forum, I think you will get people bearing out blackangel's recommendations; I know Inchworm has done salt baths for cories in desperate straits, even though she would normally tell people not to put salt in a cory tank. Maybe a salt bath might be a better option?