Scaling text in Keynote
My solution: 


duplicate the slideshow, 
resize the entire document (Document tab of the 'properties sidebar' > Slide Size dropdown)
copy the correctly sized elements back to the original doc.


Any variation of this will probably work - the key principle is that if you resize from the Arrange properties tab, objects will resize whilst text won't; if you resize the document: everything stays in proportion.

Using the accepted answer's method, I wasn't able to edit the content I'd pasted from preview (the pdf contains a rasterized png, I believe).