I believe his first release was on Rounder Records in 1990 -- the disc, "Steve Riley & The Mamou Playboys." (self-titled release). I'm not aware of another release prior to that one -- but I'll bet there's probably a cassette or two.
Hey Nick ~ If you find a copy of the "Early" one, let me (an' da rest of us) know the title and where you found it and I'll try to get one for myself. That sounds waaaay cool! I'll ask my friend, John Trahan if he knows about it or has it -- if anyone has it, he does -- he's known Steve forever and a day.
Off the subject, but similarly, I've got a live recording off of KRVS of "Wayne Toups & Cajun Creole" (before he became "Zydecajun." That dates back to 1986, if memory serves me correctly. Lots of pedal steel and Wayne's scortching riffs and syncopated arrangements -- even back then!