I go to my local touchfree laserwash for the underbody spray.
Maybe check this place out?
http://www.parmalaserwash.com/the_wash_card.htm
The one near my place includes underbody in the basic $8 package, but it seems like you'd need to get the next package up, which is $10.
It might be different between the laserwashes, but mine has a 30-second underspray as I enter. I went too one time and it stopped before it was able to spray the rear.