[JKT]

FWIW, there is a way to maintain the proportionality of a shape and its associated text while keeping it editable.

Create your shape and input your text. Go to the edit menu and select Copy As... PDF to copy the original shape, then paste the PDF version of it. Delete the original shape. The PDF can be resized and the text will scale with it. Ordinarily, a PDF would no longer be editable so if you wanted to change the shape or text in any way you wouldn't be able to. However, because OmniGraffle supports Linkback, if you double click the PDF (and because it was also originally created in OmniGraffle) it will open a Linkback window and you can change the properties, Save them and the changes will appear in the PDF in the original document.

Edit: Corrected foobar with application name!