I'm not having any brilliant flashes of insight on this, but one thing I do want to stress is that you do NOT want the spool/line close to your body (eg. held between your cheek and shoulder) when any air goes into the bag. Rick Inman recounted an incident last year where the bolt snap on his primary reg got caught in the line, and he was dragged to the surface by his long hose.
If you can get the bungie off the bag, you can hold bag and spool in one hand. Assuming you have arranged your regulators so that you can pick up your backup without your hand (and I think that's critical for your setup), you can take a breath, spit out the primary, inflate the bag and let it go. If you're using a Delrin spool, it will go up a little and then unwind down to you, and you can catch it after you've done your regulator switch back to your primary. You wouldn't be able to pull the bag out and verify you've cleared the knot, because I can't see any way you can do that with one hand, but I've done bag shoots without accomplishing that myself (she says, looking a bit chagrined).