I've considered it myself and will keep kicking around the idea as I get closer to the point where it would need to be done. I've searched on 'the google' and come up with various forums in which they go on at length about how foam + any moisture = rusted frames. At the same time I've read about how many new cars come foam filled from the factory. I think the G37 was called out specifically in one article I read.
I certainly would enjoy feedback from others who may have done it.