I have to partly disagree here. C-Mode Vserver migrations are non-disruptive provided that you are currently running ONTAP 8.x in C-Mode, which is probably not the case here. But as you have said Data Motion is the best way to go here, but it would require some pre-work to get the target data into a vFiler. Also he should be aware that Data Motion (vFiler migration) is not allowed between members of a cluster so you will need to move the data to a different cluster (or do a cf disable to temporarily allow it).
I was only talking about 7 mode. In 7 mode the only non disruptive way to move a vol that I could think of was via the vFiler. Ofcourse if the customer plans to use cmode then what you are saying is absolutely right. Infact, thanks for pitching in and providing that information.
I would also chime in here that ONTAP 8 supports a true cluster mode. The feature your are requesting is actually the number one selling point of C-Mode. Vservers can be migrated throughout the cluster easily with no outage, taking all volumes, luns, exports, IP addresses (LIFS) with them. If this is a key requirement for you you should check out C-mode.