MinWax Tung oil is really a simple wiping varnish....I don't think there's any real tung oil in it. I actually like using the stuff as it's so forgivable in the event of a mistake. I'm not sure what you mean by "messed up", but before you get too drastic, I'd try scuffing it (4 ought steel wool) and waxing it (Minwax or Johnsons) to see if you may be able to save yourself a whole lot of work. If you're bent on using poly over top, you should be fine provided the Minwax Tung Oil has set for several days and polymerized. There's nothing in the underlying product that will bleed through.
Good luck Rob