Having an area of numbness between the belly button and the incision is a normal side-effect of having a tummy tuck surgery and usually happens after a c-section as well (or any sideways incision in the lower abdomen). The nerves will regenerate somewhat over a year, but at this point you are over a year out from surgery the the areas of numbness that you have are likely permanent. Massage will be of no benefit but shouldn't hurt anything either. Best of luck!
The sensation changes you are experiencing are quite normal and are part of the design of the operation. There really isn't much you can do about it other than wait for improvement. Massage will not improve your sensation. I know you want to be pro-active, but this is the sort of thing that is a bit out of our control.Good Luck!