Well, unless they added some super secret (Awesome!) function that wasn't in Fallout 3, no you cannot add text to the top-center of the screen. The best you could do is a message - not a box, though - that pops up repeatedly until you reload. Also, with your system of if the clip is greater than 4, and ammo is less than 3, I would change that. If anything make it more like, if clip size is 2 AmmoLeft/ClipSize <= 50%, if clip size is 3-5 AmmoLeft/ClipSize <= 30%, if clip size is 5-10 AmmoLeft/ClipSize <= 25%, and if clip size is 10-30 AmmoLeft/ClipSize <= 15% That way if you have a machine gun with 30 rounds, once you have about 5 left it tells you, but if you have a shotgun with 2, it tells you after shooting one of the two shots.
If he can find the detection routines for being empty, he can go the HUD route.
I noticed that the one of the HUDs out therer...either UNIFIED or EXTENDED, dont recall which, adds a radiation meter (circle), and UNDER it is a threat meter that ONLY appears when a critter is in a certain range of you...it then pulses on screen while the critter(s) are there, and even tells you how many are there apparently.
Since this little hud thing pops up then goes away like that, Im thinking i could detect being empty if theres a routine for that, and then pop up as needed.
Anyway, Im not a hud (xml) programmer, so you might want to ask the makers of MTUI or Darnified, or the other huds.