What I had in mind, was actually adjusting the width individually for each staff system - I have a series of incipits, one on each line, that do not have the same length, and that would be easier to read if they get individual treatment.

Sometimes it makes for better reading if the staff system does not fill the whole width of the page. Is there a way to move the right edge of a staff system to some point to the left, so that the staff system is shorter, leaving white space to the right?