Become undead however you like, have the contingency edge, and drink a potion of life.

You quaff the Potion of life.
You feel as though you're burning up from the inside out! (speaking of which, you need to make that "burning up from the inside" or "burning from the inside out". I prefer the former.)
You feel yourself brought back from death's door.
Now at 1 Hp, and undead.
This just asks the question, where exactly is death's door? Wouldn't being brought back from death's door remove undeath?
Maybe change the contingency spent message to "You feel yourself brought back from destruction."
Random idea of the moment: drinking death, or life while undead, spends the contingency, sets to 1 HP, and flips the undead flag.

I see your point about the messages. Those can easily be corrected. It also makes sense for contingency to remove temporary undeath when it trips, such as from a potion. I'll fix that.