[quote=@Protoman]
<Snipped quote by mattmanganon>

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.
[/quote]

I'm pretty sure mattmanganon is right here. If you look into the lore, Innistradi vampires aren't technically undead, just cursed. The following is from the official Planeswalker's Guide to Innistrad (linked below for reference):
"Innistrad vampires are not truly undead, although they have some undead traits (such as agelessness and skin that's cold to the touch)."

https://magic.wizards.com/en/articles/archive/savor-flavor/planeswalkers-guide-innistrad-stensia-and-vampires-2011-11-02


Here's a relevant link to a discussion about the subject from Doug Beyer, a leading member of the MTG creative team:

http://dougbeyermtg.tumblr.com/post/52923451415/re-zombie-planeswalker-somebody-brought-it-up


Maro specifically answered about spirit planeswalkers, indicating that non-undead spirits (ex: Kami from Kamigawa) could, but the undead can't.

http://markrosewater.tumblr.com/post/171318331543/would-a-spirit-planeswalker-be-a-possibility-i

And to be fair, if the GM wants to allow, I'm cool with the idea, I just want the canon info in the thread for sake of clarity.