Well I was being serious in all that I said, but an interesting thing sometimes occurs when I say something that isn't quite right. My inbox lights up with a message from someone who does know the story (not always the same person, mind you) and the real information comes my way. That's what happened today, from someone who I'd definitely consider a reliable source close enough to the action to know what is going on for sure.
The note stated very much what you have suggested - maybe not because a customer doesn't like what is being offered, but rather they want to push the boundaries of what is possible. A session can be arranged with Frank Stephenson and the customer's input can be used to create something radical using the 12C chassis. Costs mentioned were in the quarter million range and up (well up...

) for such work. You could end up with something totally unique in the end if that's what interests you. Unlikely we will see this in droves, but again it reiterates the fact that with McLaren anything is possible.
I question how that might be feasible for a customer here in the USA where importation laws are fairly strict regarding body design, crash testing and the like, but perhaps a convincing story from McLaren that the vehicle still meets all applicable laws would satisfy the regulators? The real problem here would be if they messed with any of the items that affect emissions as then the EPA would have a fit for sure.
I guess in the end it would be a very similar exercise to what Jim Glickenhaus did with his Enzo by sending it to Pininfarina and having it converted into the P4/5. Many might not be quite that drastic, so another comparison might be what David Heinemeier Hansson had Pagani do when his Zonda HH was created. If this really happens with the 12C it would be pretty surprising - I guess we will wait and see.
In my mind it seemed more likely that McLaren might contract out the chassis to another firm in the same way that Lotus has been partnered with Tesla, or that perhaps someone was interested in a 1-make race series with minimal bodywork - think Lotus 2-Eleven. Neither of those ideas seem to be the case at this point so apologies for leading the discussion astray.
Still I was on to something, and if the idea seriously interests you then contacting your local McLaren retailer would be the best place to start.
>8^)
ER