I read about a brackish tolerant pleco once, didn't make note of it and can't find the article again. I believe that it was an article by Neil Monks, but can't find any mention on his webpage.
If you don't get it figured out here, you can email Neil at www.wetwebmedia.com ---> "ask WWM" and Neil will respond to your email.
Alternatively if you are looking for a cleaner crew you may want to try snails. I am trying to get that established ATM myself. I tried dripping a horned nerite (Clithon corona) to 1.005 but lost him after a couple of days. I figured that it should have been good since they breed in brackish. My next attempt will be to drip a marine nerite down and see if that works.
I agree with Charles. They recommend bristlenose for high ph rift tanks, but I accidentally killed a bristlenose by adding salt to a tank for the livebearers and shark catfish. Perhaps one of the gobies? I have heard that they eat algae pretty well. Fat Sleeper Goby maybe? they are brackish.