I seem to remember there being a big motorbike shop in Kristiansand, close to the Ferry Port, that did rentals, but it's at least 6 years since I was in Norway, so I can't remember the name or anything, sorry (but it DOES prove that they're out there!!).
I did find a website by a Norwegian biker, and it does have a link for his e-mail, the site is all in Norwegian, so I don't understand it, but perhaps it's worth you contacting this guy, with luck he might understand English, and be able to help you - from his site it is clear that he is keen on Touring at least:
http://www.no-alt-motorsykler.org/turer/namturer.html
If you go about half-way down the page I have listed, you will find this:
send meg en mail!
Which, I think, is fairly self explanatory.
The ONLY other thing that I can think of for you to do is to contact HC Travel, who carry an extensive worldwide list of places where you can rent bikes and tour:
http://www.hctravel.com/
Good luck, the demise of the Newcastle ferries is quite a downer eh??