shilsen,
My position is to use the score of the moment specifically to cover situations like ability score altering events both positive and negative. That way we do not have to massive headaches every time someone gets poisoned, buffed, drained, loses a magic item, hops in & out of a AMF, etc. We are not talking just about Rage, but Turning Attempts, Divine feats, spells per day, number of allowed AoOs, etc. I concede there are other mechanically logical answers, but I think my rule of thumb is easiest to apply across a broad range of oddball situations. Better a simple & imperfect rule than an ad hoc confusing case-by-case perfect one.
Draining or boosting the Raging Barbarian's Con looks like a legitimate tactic to me. Why avoid this "problem"?