IMHO it should work, I have done this in the past though not with SQL data. If it shows in windows disk man then the mapping is correct or the permissions error would be something else entirely.
It sounds like the permissions on the volume were rewritten at some point in the past on the original host and when that data is presented to an alternate host you are denied access.
Just an idea, can you re-present the lun to the original igroup, then mount it to the original host with an unused drive letter and inspect the permissions at the root of that disk?
To me it seems to be an issue that is entirely Windows related and not anything to do with the RDM.
