I ended up updating the iDRAC firmware to get the new "forceoffline" command.  Still in disbelief, with how much money I paid for this Gen-12 server, that I have to reboot to take a hot-swap drive offline in preparation for replacement!
I have a T320 with iDRAC Enterprise and a RAID-1 array on a PERC H310.  I would like to replace one of the array's physical disks preemptively (not in failed state, or predictive failed state) and need to have the controller take it offline before I remove it.  I cannot find in the iDRAC UI, or the iDRAC or racadm documentation how to do this.  I would prefer to not need to use OMSA.  Any way to do it?  Thanks for any help.
Take Physical Disk Offline using iDRAC/racadm