Basically...the text you'd have to change to affects the interface also affects the text in quite a few other areas of the game. The only way to get around that would be to DRAW the letters ONTO the interface, but unless you made the letters similar to the rest of the text, they might look out of place.
PLUS even if you did draw the letters the way you wanted them, they wouldn't change. For example, you couldn't draw the weapon name. If the letters "Minigun" overlap your interface, you'll have to either change most of the text in the game at once, or live with i, because if you actually drew "Minigun" onto the interface, EVERYTHING would be called minigun, no matter what weapon you chose.