Is there any way to loosen up stiffened treated cloth surrounds of a vintage loudspeaker?

Not really. Time and chemical change have gotten to the surrounds. If the cones are good, it might be cost effective to get the surrounds replaced, but it costs because it's almost as labor intensive as reconing the whole speaker. That pesky little voice coil must be hung in just the right place. With some thought and a consulate or two, it might be possible to replace the whole driver. That'd save the speakers. But call around and hit the web for a line on places or people who do that. See what they say. There are fewer speaker repair people every year. It's a shame when the foam or other surround gets stiff or deteriorates and leaves a speaker next to useless. But the speaker cone and the surround weren't engineered to last anywhere near a century or anything like that. Sorry to be the bearer of bad news. Had to retire a pair of JBL studio monitors long ago for the same reason. The foam surround was just disintegrating.