Going by the description of how the threat API works from the Omen download page here:
Omen Threat Meter : WoWInterface Downloads : Ace: Raid, I'd say what you are looking for might not be possible.
According to that description, a mob will only report threat values if someone in your group/raid actually has the mob targeted and is considered "in combat" with it. So, if you have an AOE pack, chances are pretty good that most of the mobs won't report back any threat values. Diamond appears to still be using the pre-3.0 method of reporting threat by parsing the combat log and using a threat library instead of using the new built-in threat API.