I'm shooting a runway type show tomorrow?

Some L lenses do have IS mode for tripod setting in addition to the full IS mode...it typically changes the IS to function in just one axis. Shutter speeds sound far too low, it will be a disaster, get the organizer to agree to flashes or up the ISO, watch your WB, if you can use a grey card and set custom WB with it - it will save a bunch of post editing time.