I just read this - there is a workaround, which is to create layers of nsliders representing the elements and colors to be displayed. Enable or disable staff and clef, and note to select desired elements (notes can be disabled by enabling the polyphonic mode attribute - you may need to use a clear message to clear out note data). Arrange stacked elements to create the desired display effect. For effective note opacity, layer the note nslider, with a desired background color, behind an object with the same background color (it could be an nslider), and both of those behind the other elements. Then adjust the opacity of the solitary background element.

This technique could also be used for a polyphonic multi-color nslider display.

Example (enable presentation mode to test):

I got far enough to get note opacity to work, but the note will appear to be in front only if it is the same color as the staff lines. It is possible to have note note with a different color appear in front of the staff lines, but layers would have to be crossfaded or something else to make that happen.

Also, if the note and staff line colors differ, ledger lines will take on the note's color.