Any chances to flash those phones to alternate open operating systems?
I would expect a load of them to appear shortly in the used market, so it might
be interesting to have something to experiment with while giving the devices another life.
Not really. The older BlackBerries were not powerful enough for any modern OS, whereas more recent ones (i.e., the many Android-based models) have permanently locked bootloaders, meaning that even while some form of Linux might run on them, it will not be able to without the keys.