Bar property: do not hide
As far as I know, if a part is marked as "Hide empty staves", then the only way to stop this happening in a particular case is to add a note, make it invisible (and "do not play" if you care about that). So you have something for flute, clarinet and piano (I do), want the flute and clarinet parts hidden during an extensive piano solo, but do not want either flute or clarinet hidden separately, then the hidden note messes up the Fl and Cl parts because multi-bar rests do not show properly.
I suggest a property of a bar (everyone knows what that is, don't they? Americans must understand it, because Gardner Read spends a paragraph ranting about how it's "wrong")... "Do not hide". This is an extra property and an extra checkbox, but can be implemented very simply: when checking a line for whether each bar is empty, you check the "Don't hide" flag, and count it as non-empty.
Easy to implement, but low priority, I think.