While I suspect that Crossan may be correct about the bodies of the convicts being left on their crosses or dumped into a convicts grave, as a general rule, I have sincere doubts that that would have been allowed just outside of the Upper City in Jerusalem of the First Century.  I believe that there are various purity laws that would have forbidden that practice and the Romans, though cruel, were pragmatic rulers.
Regarding the crucifixion victim's discovery, I've always found it interesting that the leg bones support the Gospel account of what was planned for Jesus; the breaking of the legs.