Hi,
As far as i know this would not be possible.
When VRF is involved, all you participating interfaces will be confined to that VRF.
Hence in your example, the VRF MMCUST1 will have its own inside and outside interface.
Similarly MMCUST5 will have its own inside and outside interface.
The interfaces on one VRF will not see the interface participating in other VRF.
So, the above configuration would be invalid.
You can do nat only for the corresponding VRF of the interface.
-VJ