>>A primary reason this could not be done on Columbia is because the payload bay airlock door had a Spacehab module assembly bolted over it. As far as I know, there was no way to leave this particular Shuttle in space, even with a spacesuit, short of depressurizing the entire cabin and going out the main door, which I am not even sure is possible.<<

When a SpaceHab module is carried, the connecting tunnel contains the airlock hatches. One closes between the spacehab and the permanently-installed airlock, and a hatch is provided on the top exterior of the tunnel. These are called "C" and "D" hatches.

NASA always allows for the possibility of a contingency EVA in the event the payload bay doors cannot be closed by remote control.

-Kim-