I'll fourth this. Unless an attack roll used for the spell, it can't critical hit. Channeling and spell storage weapons only discharge the spell on the target on any hit, and the spell simply takes effect without an attack roll. The same thing can be said about using an unarmed strike or natural attack while holding a touch effect generated from a spell, spell like ability, or supernatural ability. The touch effect affects the target (if legal) when the unarmed strike or natural attack hits.
Using a weapon like spell with their native touch and ranged touch attacks allow for the critical feature.