So am I correct in that exhaling slowly amounts to the same thing as sitting in a chair at home holding your breath (other than as you say dmaziuk, that some CO2 is escaping, which may slightly reduce your urge to breathe)?
You'll have your muscles working and burning a bit more oxygen than you would just sitting in a chair, but I expect the main problems to be the urge to breathe and slowing down.
When I do freestyle pull, I start with a good breath and swim just keeping my epiglottis open (looking forward helps with that) and letting air escape on its own. I breathe out some in the turn, to keep pool water out of my nose, and I need to inhale after 30 metres or so. Main problem is I can't do 30 feet per minute that way, more like 60 metres per minute if I go slow.