Well... I did write a Sam/Daniel friendship fic where they were forced to have sex (aliens made them do it). :) "Tangled Alliances" (http://users.livejournal.com/traycer_/78115.html) in case you were interested. They were just friends in the fic, who had to "do it", while Sam and Jack were still trying to distance themselves after "Beneath the Surface", so there was a great deal of triangle angst and jealousy and all kinds of good stuff in that story.

But yeah, if it can be done with Sam and Daniel friendship, it can be done with Sam and Jack friendship. The tricky part is convincing the die-hard Sam/Jack shippers that they really are just friends.