Well, a RAID 1 mirror is a mirror.
The disks - until they failed - had exactly the same contents.
So the Office install was on BOTH drives.
If - after the "repair" you performed the booting drive has no Office install, it only means that *somehow* the repair procedure removed it or some parts of it.
You can well (from another OS) copy the "working" Registry from first drive to second one, but the result will most probably be two separately working fine disk drives, BOTH without Office.
One of the two (very likely) will have a modified Disk Signature (automatically recreated by the Windows 7 booting with "separate" drives due to the conflict it will have found, as such one of the two disks won't have proper drive letter assignments (and as such won't boot properly until you fix/delete the DosDevices letter assignments).
As I see it your only way out (not easy at all, mind you) is to remove from the system (disconnect) the disk drive that is now booting and troubleshoot/fix (manually) the non-working one.
If all that is needed is the Office key, usually it can be retrieved by using some script ot little thrtdpart utility, however this may depend on the version of Office you had installed.
jaclaz