I think it's a nice combination of a good mechanical feature with the right flavor for the symbol. This would be nice for the combat-version of Phase as well. Sheepishly, I can't say I tested it since as a sorcerer as long as channel incant is off, looting is quick business!
My way of thinking about it is that the spell or effect might wear off when they die, but it might take a short time for them to reform into their non-corporeal state. So, their first action round after death transforms them back, and their second action round makes them decay. In that sense, it would not be a perfectly 'normal' decay. What do you think about that modification to the idea? My eyes cross a bit when I think about a non-corporeal phased dead undead creature and intuition is slightly at a loss...
