Yes, Ive seen them done like you describe ( usually using Mazda bongo seats ) personally, in a accident I am not sure if the seat hinge would hold, BUT its better than nothing I guess, as for being legal, the DVLA ( or whatever dept deals with it, I forget now ) always stated that seat belts needed to be " professionally fitted by a competent person " ( Legally a lose term )
doesn't need a headrest though
as for the VC5, just send it of to them with the seat number changed, thats it, that covers your insurance aswell by all accounts