I wouldn't do that. Instead I would fit separate solid straps from the child seat to the centre seat belt bolts and secure it that way. I take your point. You want the child to be able to see forwards between the driver and passenger seats and not get bored, fractious, or sick down the back of your neck. The best car for that is the new Citroen Picasso which has three proper, full size rear seats, each with an ISOFIX tether: www.youtube.com/watch?v=-L2mfn0r9yM . Be aware of the strength of the seatback springs on a Grand C-Max. They once knocked me out and partially detached a retina.