making an `installed-font?' scheme function

[Mark Polesky]

On -user I asked if it were possible to specify a cascade of
alternative font selections à la html/css, and I didn't get
very far.

So I'll ask here, a slightly different question: Is there a
way to implement a scheme function like `installed-font?'
where you could do things like:

(if (installed-font? "Times New Roman")
  (markup (#:override '(font-name . "Times New Roman") expr))
  (markup expr))

Even without a full solution, does anyone know a direction 
to try with this?

Thanks.
- Mark