Hypersmurf said:So your low BAB prone guy makes his Cleave attack at the same low BAB, and adds the same prone modifier.
Agreed. That was a poor example.
Hypersmurf said:The CDG guy uses no BAB, and adds applicable modifiers.
The CDG guy did not have a BAB. Hence, you cannot use the same non-existent BAB as a non-existent BAB.
Coup de grace is a special full round action. It is not a full round attack. It is not a standard action attack. It does not have a BAB. It's non-existent BAB cannot be used to do other things. You cannot Power Attack with Coup De Grace. You cannot Cleave afterwards. You cannot Fight Defensively.
You are not using BAB, hence, you cannot change your BAB or reuse your BAB for something else.
A BAB by book definition is an "attack roll bonus derived...". If there is no attack roll, there is no attack roll bonus. No bonus does not mean zero bonus.
Just because Coup de grace does damage does not mean that you get to use a BAB from it for something else. It is not a "Special Attack" (or in the special attack section). It does not state that you are doing an attack. It states that you are doing damage.
On top of that, if you get damaged while doing a CDG, you must also make a Concentration check.
"it involves focused concentration and methodical action"
"while engaged in some action that requires your full attention."
Concentration does not list CDG, but it does specify actions that require your full attention. Since CDG involves focused concentration by definition, it qualifies.
In fact, I would rule (but this is more of a house rule since it is not explicitly stated) that you cannot even do an attack of opportunity (let alone a Cleave)against others around you while doing a CDG since you do not threaten them.
"you threaten each square into which you can make a melee attack, even when it is not your action"
If you are performing a methodical action that requires focused concentration (e.g. disarm device, casting a spell, picking a lock, coup de grace), you cannot make a melee attack into an adjacent square without giving up that action and concentration. You cannot slowly and methodically be putting a dagger into someone's throat at the same time that you threaten someone else with the dagger.
Now granted, the threaten rules do not explicitly state that if you are concentrating, you do not threaten adjacent squares. However, if you look at the overall rules (AoO against you if you concentrate, threaten only if you have a melee weapon or armed unarmed attack available and useable, cannot AoO if you cannot do an action like when stunned, can lose the spell when casting defensively, etc.), the implication is clear. At least for me. I view this as an omission.
Do you allow a caster who is concentrating on casting Summon Monster I to do an AoO because he has a dagger in one hand? I don't. I think the rules imply it even though they do not outright state it. Casting defensively, on the other hand, does imply to me that you are threatening those around you if you have a dagger in your hand (since you are paying some attention to them). How can you threaten someone who you are ignoring because you are concentrating?
To me, if you are concentrating, you have no BAB because you are not actively attacking or defending. You are doing some other action. JMO on this last part though.