I've looked through all.log but nothing really points to what handles the ability to slide. Disabling the use of down on the control pad has flaws because then you can't duck, but if you condition it to be disabled on slopes only you can still slide if you were moving before you reach the slope.
Why not just do this then? Check if he is on a slope and if he is disable pressing down. (There might be some RAM address that checks if he is sliding, but I don't remember SMW's RAM to well...)
I don't quite understand what you're implying here. If I disable the use of down you can't slide but you also can't duck which I don't want. If down is disabled on slopes only, you can still slide if you were ducking before you reached the slope, still resulting in a slide regardless if down is disabled or not. Neither of these are good or practical ways to do this.