Got it.. sorta
I was looking to do this as well, only thing I could come up with is opening the stationery from the dock which opens a new email window in Mail with the template set.
To find it, go to
~/Library/Application Support/Mail/Stationery/Apple/Contents/Resources/Custom/Contents/Resources
and you should see your custom stationery in there. Drag that file to your documents side of the doc and click on it each time to start a new email with that template.