[quote=@mattmanganon]
[@Lucian] I am afraid that Spirits cannot be Planeswalkers as Sparks are destroyed at death, even if their host goes on living in some way. This is why Elspeth is stuck in Nyx and why Karn lost his Spark when he was "compleated" by the Phyrexians as it is a form of undeath. I admire the character, he looks like fun, but it is in direct contradiction with what the canon has put forth.

If you can find a source in the canon that supports your character, then that's fine, but to my knowledge, it is fact that Sparks are destroyed at death/undeath.
[/quote]

Sorin Markov's spark activated upon his transformation into a vampire. Vampires are considered undead. I would consider that at least a canon counter to the claim undead can't hold a spark.

I've taken a shot at asking the question on Blogatog to get confirmation straight from the source about what happens when planeswalkers die. I'll link back if he answers.