Paco, I really have to ask.... What would be the point in that?
I understand The hair on the back of your head is still good, but why go to the effort to keep it while the rest of your head is shaved? It's not like you would look anything decent enough to leave the house without your piece like that.
At least with shaving it all and getting a normal full cap of have the OPTION to go scalp-bald for a day or two.. Plus, it would be much easier to just shave the whole head and stick a wig on it, than shaving most of it, but being careful not to cut into your little patch of remaining hair.. Not to mention maintaining that hair, and having to worry about blending it with the piece hair...

I don't know.. Up to you..but I'm thinking a regular full cap shape would be much easier in all aspects.