This was not obvious to me, but is (I guess) a fairly well-known kernel-ish thing. Handy to have close by when you have tape drives that are turned on and off, etc.
In Tru64 this is much easier, it seems like there is an option to hwmgr to do this (hwmgr -scan scsi ???). In any case, it's nice to not require a reboot to get this job done.