Welcome to the forum!

If I understand you correctly, the issue here is Layout, not Playback.

You need a “stretchable” fermata symbol you can make wider without the symbol getting taller - right? (= does not behave like a font character which increases its height proportionally as you increase the font size)

I would create the symbol as a shape expression.

In the Expression Designer (in the main chiclet) I would make sure that the option

Allow Horizontal Stretching

is selected.

What does that mean? Will your performers understand it, without wasting rehearsal time explaining? Is there a more conventional way of illustrating the same thing?

(Just playing devil’s advocate here. You can certainly do whatever you like!)

Peter Thomsen, thanks - just what I needed.

Mike Rosen, also thanks - but that's what the composer wrote in the autograph manuscript and I'm stuck with it...

Mike, large fermatas that cover more than one note are not common, but do occur in Mozart and Schubert and other composers. See:

https://notat.io/viewtopic.php?p=100&hilit=fermata#p100

Will your performers understand it,...

No, they won't!  I have to go with Mike R here. There has to be a better way. It is always best to make things as clear and as simple as possible, IMHO, of course.

I agree that obscure notation is generally not a good idea. But the large fermata is actually fairly self-explanatory when you see it in context. It means the same as a single note fermata but applies the fermata feeling to several notes, which are played and held freely as a group.  I don't know of a  more commonly-used notation that really substitutes. A piacere, or ad. lib. comes close but doesn't give quite the feeling of expectancy of a fermata. 

I don't think Mike R and myself disagree so much on whether it is a good music expression or not but whether it will require explaining to the musicians.  I have never seen it and I would not know right off what it meant except it was a printing error of some kind. If I wanted something like that I would probably use "directed" and explain it to the music director in the notes.

I agree completely that having to explain notation is generally a bad idea and often indicates a notational failure on the composer's part. And as you said, it may be necessary to explain it.  However, it may also be fairly clear to many musicians without explanation.

Here is an example from Schubert in the Wiener Urtext edtion that I provided in the notat.io thread I mentioned above:

Yes I can see that. I am not a piano player or at least not one that anyone would care to listen to, so my thoughts were towards concert band or brass and woodwind ensembles. In a piano piece it looks like a legitimate marking and I would not need further explanation. I would never use it for any of  my applications. (I don't think anyway.)