No, I'm arguing direct manaburn. Dragons get manaburned and they're way more corporeal than Urza. Being incarnated energy, if he loses the mana to manifest, he's outta there.
I know, I was just making a joke. >_>
I still don't think that's likely to ever happen, though, considering planeswalkers can regain mana just from planeswalking.
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"The courtroom is the garden of holy judgment. Those with lechery in their hearts should leave this sanctuary at once!" ~Franziska von Karma