Glad to see this thread pop up, some good solutions here.
We (me & my trusted mechanic) put a 195/45R17 on my stock rear K1200RS wheel. It was a real bear to get on, but it seated perfectly fine & haven't had a problem with it in over a year of it being on there. The stability of the rig went way up, but the speedo is off a solid 10% above about 25-30 mph.
I had the opportunity to check the odometer parallax with a GPS this past weekend - at 244 on the bike's tripmeter, the GPS only registered 187 miles! According to my rudimentary math skills that's a 1:1.3 variation. It's somewhat for this reason that I'm thinking of getting Claude's rear wheel adapter, plus one benefit of that is having more choices of tires to run on the rear.
As far as fit went, we tried several tires, 215, 205 & 195. The 195 was the widest that would fit without rubbing the swingarm, muffler or both. My mechanic suggested using the spacer from a K12LT to get a little more clearance from the swingarm, but that didn't necessitate using longer bolts to secure the wheel.
I've been keeping my eye out for an R12CL wheel or even a 5.5" wide 17" diameter BMW wheel - they used the same bolt pattern on several 17" wheels.
PS Claude did an amazing job putting my rig together ... can't thank him enough!!