You can use Compressor to set up a cluster over your LAN, though I haven't done it for rendering but have used it for transcoding the final product to a format suitable for web streaming.
But, the nMP and FCPX are likely to end up working faster together than doing something like having the nMP be part of a cluster since FCPX should be getting an update to use the CPUs and GPUs for rendering and compression.
The RAID will need to be connected to whatever device is the master in the cluster, I'm not aware of any software that would let you have the rMBP work as a client for a headless editing station (other than using remote desktop).
There's no way to directly connect the two, though, such as via TB.