Sorry, I don't have the answer to your question. At first I was surprised to see that KeyFramedMotion has been implemented at all. The docs really need an update!
However, I remember in SL keyframed objects can have a physics weight of up to 64. Now, in Opensimulator, I don't know how mesh will be counted in this context, but according to the error message you got it might well be the source of your problems.
Note (added 10.2.1015 15:27 CET): this needs further investigation as I just made a simple object of combined prims and mesh with a total LI of 130 and that works fine.