Ok, I'll admit I suck at the grapple rules. But to me, the Sage's answer is wr___ ... er, does not make sense.
Under "Starting a grapple", there is wording on BAB.
Under "If you are grappling", there is wording on BAB for ACTIONS.
--Activating a magic item (included for illustration - this is an ACTION that you cannot do more than once per round - its not based on BAB.)
--Attack your opponent - (attack roll vs AC) includes unarmed attack, natural attack and light weapon. Also, in this paragraph, two weapon fighting is excluded.
--Damage your opponent - (grapple check vs opposed grapple check) allows an opposed grapple check to do unarmed damage.
--Pin Your Opponent, Break another's pin, and Use Opponents Light Weapon are also listed, but all have similar wording about replacing an attack. All are covered under the general "if you are grappling" wording, directly above.
To me, this covers all the possible attacks before or during a grapple. Did I miss something? In all the cases above, it's based upon BAB - nothing about a creature with natural weapon attacks, except of course using a natural attack in place of a BAB-based attack.
In addition, I noticed that higher-level monks get "more" BAB-based unarmed attacks than other characters, so they get to do more BAB-based stuff during a grapple than other PC characters. As for monsters, I suppose one could design a monster which gets +9/+9/+9/+4 for BAB-based attacks (like a level 12 monk), and then it would get to do more stuff during a grapple, too. Since monsters usually can only exceed normal BAB attacks by taking -5 penalties, etc, the CR would need to be adjusted, too.
Comments welcome, of course.
--------------------------------------QUOTES FROM SRD BELOW
Starting a Grapple
To start a grapple, you need to grab and hold your target. Starting a grapple requires a successful melee attack roll. If you get multiple attacks, you can attempt to start a grapple multiple times (at successively lower base attack bonuses).
If You’re Grappling
When you are grappling (regardless of who started the grapple), you can perform any of the following actions. Some of these ACTIONS take the place of an attack (rather than being a standard action or a move action). If your base attack bonus allows you multiple attacks, you can attempt one of these actions in place of each of your attacks, but at successively lower base attack bonuses.
[then the srd lists the ACTIONS you can perform while grappling]:
Activate a magic item: [self-explanatory]
Attack Your Opponent: You can make an attack with an unarmed strike, natural weapon, or light weapon against another character you are grappling. You take a –4 penalty on such attacks. You can’t attack with two weapons while grappling, even if both are light weapons.
Damage Your Opponent: While grappling, you can deal damage to your opponent equivalent to an unarmed strike. Make an opposed grapple check in place of an attack. If you win, you deal nonlethal damage as normal for your unarmed strike (1d3 points for Medium attackers or 1d2 points for Small attackers, plus Strength modifiers). If you want to deal lethal damage, you take a –4 penalty on your grapple check.
Exception: Monks deal more damage on an unarmed strike than other characters, and the damage is lethal. However, they can choose to deal their damage as nonlethal damage when grappling without taking the usual –4 penalty for changing lethal damage to nonlethal damage.
Also: Pin Your Opponent, Break another's pin, and Use Opponents Light Weapon (not quoted here)
Also, under Monk: The resulting modified base attack bonuses are shown in the Flurry of Blows Attack Bonus column on Table: The Monk.