The earliest I can come up with a picture of someone other than a baseball player having a pair of pants with a belt loop on them is 1905 and is a rodeo cowboy in Denver. (Picture 12615 of the Butcher Collection)
Now there are two guys wearing belts, but only one appears to be wearing a pair of pants with a belt loop. I've seen the belt with out the loop before at an earlier time, but not the loops.
I'm going to go over those pictures with a fine tooth comb late, but I doupt there is enough pixels there to show us if they really have loops or not or if there is really even a belt there.