heat is moved through an offset by using the tendency of hot air to rise out of the chimney and the resultant air flow pulls the heat from the fire chamber into the smoking chamer. Hence the importance in having the balance between exhaust and intake capacity. Also why extensions to the chimney can result in lower hot spot temperatures and more even convection heating through a chamber. Vertical smokers such as UDS types do not require long chimneys as the vertical chamber creates the convection draw itself. All of this is not disimilar to a house, wherein, you can create up and down drafts in stairways if the house is not properly vented.
I can see what you are thinking as one way to achieve your goal of having longer smokes available to you. At one time I was giving a lot of thought to having a Smoky Joe connected to the bottom vent of a old style Weber Kettle and using the Smoky Joe as an offset firebox. Just not sure why I would do that.
"perhaps...but then again...maybe not..."
careful there son, those ribs are boiling hot...
(='.'=) Here there be bunnies...
Pacific Rim BBQ
Bob's Brew and Que