Wide, very low tack masking tap will protect the top from radiant heat and scratches as I loosen the end of the fingerboard:
My heat source is a block of aluminum heated in the oven at 400 degrees Fahrenheit. I frequently the electric heat blankets, but for this job I wanted the most precise control. I know that whatever happens there'll be a limit to how hot the wood gets!
I'll feel underneath to monitor the heat penetration, and when it's quite warm to the touch, I'll start testing with my knife. My knife is a regular flexible putty knife I've carefully ground to an extremely fine tapered tip. I ground it by simply rubbing it on flat surface covered by 320 grit silicon carbide waterproof paper. It took about 2 hours to get the blade to just the right flexibility, so that when I press it down to a flat surface it makes a smooth contact all the way out to the very edge. I keep the blade sharp and polished by buffing it with emery compound on my buffer at least once a week.
With the block still in place, I'll start lifting the fingerboard. Once I'm confident that the glue joint is parting cleanly, I'll remove the block and loosen the fingerboard right up to the 13th fret.
Once the fingerboard is loose, I'll pull the 13th fret and drill a 1/16" hole through the fret slot and into the pocket at the end of the dovetail joint:
Now I have an access hole for my steam needle
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