It doesn't happen to me too often to me thankfully but I can imagine it being a major pain for people with very large datasets to copy to tape.
It appears that you cannot unlock a library to change tapes etc. whilst a job is running. This seems to be a self-imposed limitation (it's not unreasonable to want to change tapes whilst long jobs are in progress - BExec has allowed it for as long as I can remember) and means that you can only swap tapes when jobs aren't active which is rather restrictive.
In my case, the library in question isn't even currently in use! We have two libraries connected. Library 2 is currently running a job (which I'm now having to cancel) & library 1 needs a tape swapping but is currently locked, even though it isn't even in use :-/
[New Sig: PLEASE get GFS tape support for incrementals!!!]