This is an interesting one. I used to do the same thing - removing and adding the rdm on the fly with this command:
vim-cmd vmsvc/device.diskaddexisting $VMID /vmfs/volumes/DATASTORE/vm/rdm.vmdk 1 0
I have not used this for a while, and today when I checked this is not working any more. "Reconfigure Failed" is what I got...