This thread has been awesome. Wonderful early morning reading at work.
The only concern I'd have about using a steam cleaner is ensuring that the couch dries fully. You don't want to get it too wet or else it'll just never dry - and that'll contribute to the already musty smell. Especially if you're in Vancouver where the RH is pretty high on the best of days. I'd personally start with the baking soda, liberally covered over the cushions, let it sit for a while, then vaccuum up.