I ran across your website...

http://www.oldnewark.com/memories/newark/mcgrathpaddy.htm

...and your reference to the term "pie wagon" as a synonym for the other term for a police van, "paddy wagon", the latter being the term I recall from my own youth (not through personal experience, thankfully).

I can tell you at least one more place I have heard the term "pie wagon", which suggests it was a wider-spread term at one time, but now out of fashion, as is "paddy wagon" itself.

It's in this odd bit of dialogue from a W.C. Fields movie in the 1930s, probably "Poppy", but I don't recall for sure. Fields often wrote his own scripts, and he was from Philadelphia, so maybe it was an Eastern Seaboard term:

Actor: I understand you buried your wife recently.
Fields: Yes, I had to. She died.
Yes, my wife was killed, in Upper Sandusky.
Run over by a pie wagon.

My impression would be that the original "pie wagon" actually delivered pies, and the police panel truck must have reminded someone of prisoners being delivered to the station like so many pies.

Some other references:

http://www.armlesstigerman.com/intro/pulp/slang2.html

http://www.hubcapcafe.com/ocs/pages01/f1002301.htm

http://naid.sppsr.ucla.edu/coneyisland/articles/food.htm