The workaround for one or two measure repeat symbols is to add them as text (shift-X) and hide the bar rest with “Hide rests” or setting the opacity to 0.
Unfortunately this doesn’t work in percussion staves where you have these measure repeats very often. I’m not able to hide the bar rests with the methods described above.
Is there any other way to get rid of the bar rests in percussion staves?
No, unfortunately rests in percussion staves are entirely non-editable. You could somehow use instrument changes to force Dorico to switch to a pitched staff for the empty bars in the percussion staff, and then hide those bar rests in the usual way…
OK, I tried your suggestion. But how can I get rid of the clef for the pitched instrument after the instrument change?
Perhaps use an invisible clef on the pitched instrument to start with?
Found a “solution” for it!
Create a note instead of rest, select it in Write/Engrave mode, open the Properties panel, enable “Custom scale” and set it to 1 (optionally enable also “Suppress playback” to “ignore” that note). And that rest is finally gone!