Hi,
I know this thread is a little old now but after reading it you could have used the config.vpxd.filter.rdmFilter in vCentre in order to overide the detault behaviour which is to hide already mapped RDMs. Once you could see it then you could map the RDM to a VM and then re-apply the filter.
Instructions on how to apply the filters can be found here http://www.yellow-bricks.com/2010/08/11/storage-filters/
Regards
Jeremy