You mention that there is now nothing in /dev/rmt.
Numerous times I have seen a patching operation wipe out network gateway and default router files information but I have not seen it affect tape library configurations. However I guess that it's possible that the /dev/rmt 'special node' files are all that is missing. Do you have any other backup of this system other than on the library that you cannot communicate with? If so, you could try restoring /dev/rmt/*
If it's only the /dev/rmt device node that's missing you might just be able to communicate with the library again.
What backup software are you using? Do you know how the tape library was configured in the first place and where its configuration files are?
I know that NetBackup can lose communication with a library but NetBackup provides a button within to rescan for devices and doing that normally fixes things.
As others have mentioned, recheck firmware levels, et al. Also, no harm in continuing with the full recommended patchset.
Also, as someone else has said, what about the library device firmware level; perhaps that's too old.