U Pug - thanks for the ideas - It's pretty much exactly how I do it, except -
I've currently just got a single ended bolt snap tied into the centre of the spool... simply because it was the first snap I found in the junk box when setting up a spool. I'll swap for a double ender and have a play.
I've not see a video of anyone sending up a bag... but I'm fairly familiar with the concept of being able to release the spool and watch it spin in front of you - was a bit sceptical when it was first suggested, but pleasantly surprised when I tried it.
Most of the times I've deployed a bag there has been a combination of current and wind - if I tried locking off the spool and letting it go like you suggest, it would vanish in seconds. The drag from the bag on the surface is often enough so that my buddy has to hang on to my BC so that we stay together...
The place I dive most often is at the mouth of a large bay, diving along the walls of what was a river bed some 10 000 years ago, but is now submerged to 20 - 50 m. The bay is probably 50 km's diameter, so there is a lot of water running through with the tides. A lot of dives can only be done on slack water - which actully means descend at slack, and the tide is running by the time we ascend.
Makes life interesting at dusk on an outgoing tide, with 8 or so buddy pairs being rounded up over a large area and time by a live boat....
Mike