Nothing really available.
There was a member here (sorry, forget who, I'll try a search later) who lengthened their passenger section. It would take some work seeing as you'd also have to move the sissybar back, but could be done.
Maybe do what others have done... keep an eye on ebay for an extra seat and go custom from there.
AKA: Tim & 'The Adventure Cycle' VROC #24567, NEVROC, SteelCity VROC
"When life throws you curves,
Aim for the apex."