There is a possibility that VW have changed the airbag deployment process for the MK6 to improve safety (i.e. the way the bag is folded, the force at which it fires, and the vent holes in the bag which allow it to cushion your contact with it), in which case it might be compromised if you fit it in the MK5
I would have thought it would fit though.
Also not sure if the MFSW functions would work - isn't there a different range of wheel functions on the MK6 wheel?