The one I am referring to was a man who served in that war, and became famous as a political agitator in the 1970's by throwing blueberry pies into various California politicians' faces, including then-governor Jerry Brown. The comic strip Travels With Farley made fun of him by having him describe his service in WW1, where he used blueberry pies as weapons "to embarrass the Huns into surrender." I am not making this person up, but I don't know his name or anything else about him. Does anyone else remember him? (Obviously I can't Google him on such spotty information.)

I can find articles about Gov Brown being pied, by a Aron Kay, but that doesn't seem to be the vet you're speaking about. There is even an article on Wikopedia about pieing as political statement.
http://en.wikipedia.org/wiki/Pieing

I grew up with Jerry Brown as my Governor, I sure don't remember anything about him being pied, but of course that was 30+ years ago.