Did you ever crack this?
I'm trying to create a button as well with focus management that affects its style and i'm hitting this barrier too.
You can set the font by including a child font object in the XML. You can set the “role” of the font object to the value of the font setting you want to use. Here is an example of how to do it with a label object, but it should work for a button as well. From the description, it sounds like you will need to set the role to “focusedTextFont” or “textFont”.
<Label
id="Text1"
horizAlign="center"
vertAlign="center"
width="600"
height="60" >
<Font role="font" uri="pkg:/Fonts/YOURFONT.TTF" size="32" />
</Label>