I'm not sure why the ownership restriction is needed. Ideally, such an interface would let a process tell the kernel "I'm allowing somebody else to send messages to me". That is, the copy would occur only if a copy_to_process() pairs up with a copy_from_process() and the buffers match. In effect, the processes would negotiate a communication channel, it doesn't really matter who owns them. Though yes, I can see that looking at the PID isn't enough to prevent issues, perhaps another authentication scheme is in order?
Besides this, it's really good to see IPC performance improvements in the kernel.