These affect the numbers or "Resisted!"/"Miss!" text that appears when something is attacked.
combatbubble_startheight: Apparent range -10 (bottom of screen) to 10 (top of screen). Affects the starting point for the text. Unaffected by any letterbox settings.
combatbubble_use_shift: No apparent effects to changing.
combatbubble_use_base_loc: No apparent effects to changing. Probable that I don't have the correct syntax.
By the way, it's the names, not the combat bubbles, that gets shifted when you alter cl_letter_box_position. Demonstration:
cl_letter_box_position 0.5 (default):
cl_letter_box_position 0 (all black portion at bottom of screen):
(That's not a problem by itself, but targetting gets screwed up if the camera isn't perfectly level.)