To follow up on Greybeard's method (I have a cigar box full of rolls from my father, taken in the 40s and 50s stored the same, but at room temp) - How about sandwiching the film in acid free board as you suggest, but inside the board, also inside individual folds (both sides of the film) of glassine paper? I've been told that this is a good material for shielding fiber prints when flattening just as you describe, which works well for me (for prints).
I can test it on film, of course, but I've never really known the composition of glassine material, or it's possible long term effect on emulsions. The heat, as you state, is just enough to feel warm to the hand.
I have negatives that were stored for 50--60 years in glassine envelopes, and they don't seem to have suffered from it. I mentioned silicone release paper because it is very smooth and is pretty much guaranteed not to stick to anything that might have gotten onto the film. If you aren't in a hurry (I needed to know quickly whether the film had anything of immediate interest) you could put the film in a folded strip of either glassine or release paper, and just store it flat under pressure for however many days, weeks or whatever that it takes to flatten out. Heat would accelerate the process, but isn't otherwise necessary. Agfa used to make strip-form glassine sleeves that were assembled like a Venetian blind, and I found one that I had left in a book for a few months. The negatives were just as flat as could be (this would have been 1970s-vintage Tri-x 35mm).
Thank you for your reply. Actually, I have taken a sample roll from the group I mention, put it into a long Printfile sleeve (the length of the roll). I then took the sleeve and wound it "backwards", emulsion out against the curl, around a small tube (like a paper towel core), left it for months, and when removing it, it returned almost immediately to the curled condition. I assumed flat would do no better.
I'll try the heat.