It's very doable, more so than spme others have posted, no adjustment necessary:

I have a B17 on my flatbar, and a 27.2 seatpost on both it and the MTB. If I want to go for a jaunt on the MTB lasting more than say an hour I just whip out the seatpost on the flatbar and pop it in the MTB. A quick setting for height (easy if you have a mark for both bikes) and you're off.

You could even do same if the seatposts were different by using a shim.