Your request wouldn't be, by any chance, for a photo/press type purpose would it Tim?
If so, you will be wasting your time as the marshalls are tasked to specific areas to patrol and cannot deviate from those areas. They are also limited to 2 peeps per RIB.
I believe a forum member has already been approached by "the press" and offered money for a place on his RIB which he sensibly declined
If it's for pure sightseeing, why not bring your own boat out for the day and join in the fun with everyone else.