Is there a canonical way of placing the scoring meeple to signify having gone round the scoring track?
The official, pre-expansion rule to keep track of 50 points was to lay down the scoring meeple like a farmer.

Start of page 4 of the official rules (pdf):


  Note: if your score passes 50 points, lay down your scoring meeple to show your 50+ points


Of course, any way to mark 50+ or 100+ points that your group agrees on and that can be clearly distinguished from each other is fine.