OK, now I'm safe at home with my handy-dandy Player's Handbook. (Hard to cite page numbers in the SRD, you know.

) Page 132, Table 8-8: Attack Roll Modifiers. On the row 'Defender grappling (attacker not), the Ranged column has +0 with a double-dagger superscript. The referenced footnote states: "Roll randomly to see which grappling combatant you strike. That defender loses any Dexterity bonus to AC."
The Bull Rush rule referenced below my post is on p. 136, the last two sentences of the 'Initiating a Bull Rush' section (right above Table 8-13: Common Weapon and Shield Hardness and Hit Points, which breaks up the Bull Rush section over pages 136 and 137):
"Any attack of opportunity made by anyone other than the defender against you during a bull rush has a 25% chance of accidentally targeting the defender instead, and any attack of opportunity by anyone other than you against the defender likewise has a 25% chance of accidentally targeting you. (When someone makes an attack of opportunity, she makes the attack roll and then rolls to see whether the attack went astray.)"
Neither of these rules makes any reference to the possibility of the grapplers/bull rusher and bull rushee being of different sizes. I imagine that the size modifiers to AC are supposed to be handling this in an extremely abstract fashion ...
One could reasonably extend the bull rush melee AoO rule and the grappling ranged attack rule to the general case of attacks against a defender who is sharing a space with an ally (including to the reciprocal cases of ranged attacks during a bull rush [readied actions, those would have to be] and melee attacks into a grapple [when neither opponent is pinned, I'd suggest]).