Hiding staves which are otherwise set not to hide

I have an opera score (one flow) in which I don’t want any of the wind, brass or string parts to be hidden when empty except during a couple of recitatives accompanied only by piano. What’s the best way of setting this up? If I set all parts to be allowed to be hidden, I have to find a way of unhiding a lot of them throughout the score. If I don’t allow these parts to be hidden, they will also show up in the sections in which I only want piano. If I separate these recitatives into separate flows, I lose the cautionary key and time signature changes.

Writing/engraving an opera without the benefit of Dorico’s Flow mechanisms seems a terrible waste of energy to me.
After experimenting a bit, I think I actually found a trick to show cautionary key and time signatures at the end of a flow.

  • position the caret at the final barline
  • create a key signature if needed. Dorico will add a bar.
  • create a time signature in this new bar, but give it a short pick-up (e.g. ‘3/4,0.125’). Dorico will only show the pick-up.
  • select the bar rest in the last empty bar (in all parts/staves) and Remove Rest to make the dummy pick-up bar as empty as possible.
  • set final barline for current flow to none (Notation Options)

The invisible pick-up bar will leave a gap of about 1 space after the cautionary time signature. If this bothers you, you can reduce it by adjusting note spacing. Nudging the invisible final barline to the left will move the time signature to the right. Unfortunately, this has to be done on all layouts.
Using the pick-up bar trick is essential, because a regular bar will always show up as a whole bar rest in parts where multi-bar rests are consolidated.