Yes, for those that wondered, I am familiar with the class system. This is purely theoretical. I like it, and the cross class skills they've made available. I admit, I'm still just pushing into end game on one character on my legacy account. But I thought it'd be interesting to pose the question.
I've considered the mage tanks in this game, and actually off tanked with Mini Titty on the side. But that isn't the mage tanking, so much as it is the pet. God forbid Titan doesn't aggro, and you get eaten alive.
No, the way I see it, since damage can easily aggro enemies and there are some cross class skills to draw attention, it is possible. Practical is another matter. Until you get a job, you have healing, protect, stoneskin for spells, and access to cross class skills to increase defense, parry, evasion, and healing with the marauder's bloodbath ability for physical damage.