The following message was sent to Swen, and unfortunately, it will not be implemented as a surprise, it will just be implemented.
"Sorry for two PM's in one day, this idea I want hidden and made as a surprise if you like it enough to implement.
(…) fire can burn the skin off a zombie and they become a skeleton. When they became a skeleton, their traits and attack methods changed to that of a skeleton. Their HP remained where it was from the initial attack of the fire or previous attacks. The fire would simply burn off their skin over a few attack rounds (…)
Since so much detail has gone into this, that would be interesting to have implemented."
Thanks to this successful Kickstarter, we can give monsters more interesting behaviours, more specific attacks and powers (and weaknesses), so why not create this effect for the zombies? As it turns out, an unknown scribe had already reported on this in his "Source User's Guide To Adventuring". What a coincidence!