I've had this with black locust as well. Smaller trees that died spontaneously have this quite regularly, and have rather loose bark.
I tried to coerce a few young trees (up to 15 cm diameter) to do this for me by ringing them with a chainsaw a year ago, cutting through the cambium layer so as to impede the sap flow from leaves to roots, thereby forcing the tree to slowly wither and die. I had hoped that the sapwood would undergo the same slow transformation and turn into heartwood (sapwood becomes heartwood because the xylem vessels stop functioning, and secondary metabolites are then deposited in the vessels. But it didn't work out as I had hoped. It seems the bark really needs to loosen from the sapwood to get this effect.