for a start you dont need to split the chain to take the rear wheel out of a 20", just asking for trouble!!!
and it sounds like you want to make summit like sandpaper but on your rims? yea.....
well have you ever tryed to use sandpaper to sand stuff before if you ever have you would have noticed that it wears down REALLY fast and then goes crap!!!
so just imagine how longit would last on a rim..... not more tan 10 mins (Y)
and if you think about it when you grind you remove some material and the bits you leave behind are they bits the pads grips on. these bits are part of the rim, so they are going to be stronger than sand which has been glued to your rim
cutting slots in your rim with a hacksaw blade works and doesnt wear off for a long time, dont go more than 1mm deep :-