[racket] Scribble: exact-chars in multiarg-element

The 'exact-chars style property works with `element', but not with 
`multiarg-element'. (I can see why, in scribble/latex-render.rkt.) Was 
there a reason for this? I need to render something like

\setbeamerfont{myfont}{size={40}{44}}

and the braces around the numbers are being escaped, which of course 
does the wrong thing. How do I get behaviour like this? Thanks. --PR