OK, I found a workaround procedure to get my desired result. The idea behind it: Where no rests should be, there must be something in the same voice.
To achieve this:
- Cues with rests before or after them must NOT be “extra up-stem voice” but just their normal voice (e.g. up-stem 2).
- for bars just with slashes (slash noteheads, stem -2 7/16), just select the slashes and use Edit | Percussion | Set Voice to put them to the same voice as your cues (if you do this globally, you cannot have slashes and cues in the same bar anymore).
- in bars with stemmed notes of another direction, you have to add a whole note somewhere where it does not disturb the rest of the layout, put it to the same voice as the cues and then hide it using the property color and opacity 0. If you want to hide quavers, you additionally have to hide the stem (adjustment -2 7/16).
Make sure that your stem and notehead opacity adjustments show up in all layouts you want them! I recommend building an example showing up correctly in all layouts first and then copying this as far as possible (this will copy properties for each layout with it).