I actually did not know you could do this. But I was speaking with a customer the other day who does this. He is using his mental ray standalone licenses to get 16 CPUs help out his render regions (there is no limitation on the number of ray3 “satellites” that you can use).
This is an example of “standalone” distributed rendering, which is like “satellite” rendering, but with ray3 and mental ray standalone licenses.
You just have to fill in your ray3hosts file with the names of the ray3 slaves.