I have been to Laos several times, and I have never seen a sports bike there. It's a wonderful country and absolutely fabulous for off-road riding, and hence most rentals have XR250/Baja/KLX on offer. As mentioned Jules and Jimoi are both great guys that will be able to help you with that.
If you desperately want a sports bike, imho you will have to rent in Chiang Mai. It's a little complicated and requires some paperwork (e.g. letter from the owner), but it can be done. The one guy in Chiang Mai who regularly arranges this is Dang, but he does not have sports bikes. The next best bet would probably be Gung from Mr Mechanic, and if that fails ask Tony from Tony's big bikes, I think both of them have 250 Ninjas and ER6's.
Also, have a look at gt-rider.com and the forum there, maybe someone can help you out.
Come to think of it, Rt 13 in Laos is great, but Chiang Mai - Mae Hong Son and the minor roads in that area also have a lot of twisties and beautiful scenery. If you haven't ridden that yet, that may be a lot easier and you'll also have a great time.