… The instrument line is on a hidden staff in a Choral score …

An idea:

To get a Choral score without the instrument line, create the Choral score as a new, linked part which does not include the instrument line.

Since the instrument line is not included in this special, “Choral score” linked part, you can create multimeasure rests in this special linked part.

In a score with only one instrument, I cannot figure out how to make a multimeasure rest bar that is only one "physical" measure long.

I tried to:
Mark, say, 5 measures. 
Right click in the marked area
Select Multimeasure rests
Select Create.

That gives me a rest bar with "5" written above it. But the bar is still 5 measures long, so that no space is saved.

Please, give me a rest :-) 

Unfortunately your description is not clear to me.

… that is only one "physical" measure long …

By your word “physical”, do you mean playback?

So that the audio plays only one rest measure?

A multimeasure rest is not supposed to affect playback; 5 rest measures should still play back as 5 rest measures.

> By your word “physical”, do you mean playback?

Thank you for your reply. You are right it is not very clear.

With "physical" I meant the space it takes on the paper. I could'nt make a multimeaure rest of 5 shorter (in cm) than 5 ordinary measures. 

However, the problem was solved. 

Best regards,
Mathias