Is it worth fitting an oil temp guage to the oil drain from the turbos so that you can see when they've cooled down enough?
Presumably once all of the internal component heat has bled out and been carried away with the oil, a marked reduction in oil drain temperature would be seen (then you know it's safe to switch off) ... ?
On a related issue, I did once hear about someone fitting electric oil pumps to a turbo engine; the oil pumps kicked in for a while when you turned the engine off to allow the turbos to cool properly. Anyone else heard of that?