My steam generator is a small pressure cooker to which I've added a hose fitting. That way I still have the pressure relief safety valve in place. When the pressure gets too high, the valve lets off some steam and my hose won't blow out. Although I'm steaming the joint from the top, I get good penetration of heat and moisture immediately through the joint, thanks to the 1/8" gap at the end of the fingerboard.
Look below the fingerboard, and you can see the cloud of condensed steam emerging from the edge of the fingerboard from frets 12 to 16. The steam is shooting out almost all the way across the top and condensing on the finish. The steam is HOT as it comes out, so I'm wiping furiously with clean cotton rags to keep the heat and moisture from damaging the finish.
For more modern instruments I have a fixture to press the neck out while I'm steaming. This guitar is a very valuable and delicate one, so I'm taking my time, steaming more slowly and removing it without using my clamping fixture, which gets in the way of the view and wiping effort.
By waiting for the steam to do its work, I'm able to remove the neck without any finish damage:
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