"If Iraqi leaders were to offer to trade information for asylum, should we take them up on it? What crimes could be overlooked, and what crimes could never be overlooked? What information would be valuable enough to make it worthwhile? Saddam's current address or tomb? The nuclear bomb factory? The location of Michael Scott Speicher?"

I think that generally speaking, the best we should offer is not to prosecute for war crimes and any prosecution from US and/or UK- and going to bat against any prosecution from any new Iraqi laws.
This would dependent on full disclosure (but the person could sell his story to highest bidder:). Offering citizenship and enough money to live on could also be part of it- but it's probably more of a matter of allowing them to keep some portion of their personal wealth. It's possible that if they come clean they could possibly live in Iraq- though being part of political process should be limited or excluded.
And of course any deliberate lying would void the entire agreement.