That crack will never close with humidity or dowsing with water or any other potion. The beautiful back is crafted from squirrely grained Maple, maybe even crotch wood. It may continue along it's cracked course or it may be mostly done, no one can say. Splinting could turn out rather ugly but here is an idea to consider. The Chinese came up with a clever way to mend broken pottery. The ceramic vessel would be glued back together and then the cracks and missing bits would be filled and then gilded. The gilding forgives the visible damage and even enhances and beautifies it.
Looks like you're in for a re-finish anyway, why not patch and stabilize it from the inside and then fill the gap with tinted epoxy? Level it and re-finish. Again, no guarantee that the crack won't keep moving but short of carving a new back, you're options are limited. Good luck, I would be interested to see what you come up with.
That's a good deal more than a crack - there's some serious reshaping to do. One standard sort of repair is to make a plaster cast of the back as it is, correct the shape of the plaster, and mold the back in place using the corrected negative plaster mold. Heat, pressure, interior laminations, etc., can do wonders there.
If you're in the mood, follow this link to an extensive blog-style article about a similar arched instrument reconstruction, where you might get some ideas of interest:
Jim, please post some pictures of you're repairs to the L 5 back. I'm really interested to see what you come up with.
Looks like that old Martin was dragged around like a Raggedy Ann doll. I get a certain satisfaction from resuscitating what others would toss, just a sucker for a hard luck case I guess.
Jim, if I am not mistaken, this is a flood guitar?