Is it possible to increase the typeface size to above 6?
 

Either that or use your own fonts?

I find myself needing some truly huge typeface for a UI I'm working on.

Is either possible?

ingame command: /chatfontsize <0-5> where 0 is near-invisible in any resolution above 800x600, and 5 is so huge I can't read it in1280x1024. In xml files: <Font>(0-5)</Font>; same sizes. 3 is usually good for more purposes, 4 or 5 only if as you said you want huge.