Simile Mark as Chord Symbol (for either 1-measure, 2-measures, or 4-measures)
as reported here: https://musescore.org/en/node/12571
The percent sign with two slashes is used to indicate that the chords from the previous two measures should be repeated.
MuseScore 2.0.3 currently allows the percent sign to be inputted above a particular chord-rest:
Not quite sure the best position to display the percent sign...I would think the middle of the measure would be best to communicate that the entire previous measure's chord symbols are repeated. And for two-measure and four-measure chord repeat, would be best if it displayed exactly over the barline by default. Also the simile mark should have the slash at about 45-degree angle, as opposed to this percent sign which is a little too vertical.