Ribtec did do a 585 or 6m RIB with the Volvo 105hp engine some years ago which was featured in several magazines. I think it went to a Yacht Club in Suffolk cos the last time I saw it was at Felixstowe Ferry a few years ago. That engine has been superseded by the new Volvo's & the 1.7 Mercruiser etc etc.
AFAIK a diesel engine is more expensive than an outboard to maintain. Of course this is just received wisdom and hearsay so in true rib.net style I will state it as an absolute fact and get into a massive flame war with anyone who dares disagree with me!
(But there is some logic about an outboard being easier to access than an inboard diesel that would lead me to think that I am speaking somewhere near the truth.)