While you might be able to connect another SATA device internally you still need some where to mount the device.
You risk interfering with the systems airflow causing the fan to kick in more often, besides the risk of causing something to short on the devices housing. And lastly, you void your warrantee or even a parts exchange (via Apple) if your logic bd. dies.
What are you planning to install? That you can't do externally? With a lot less hassle or risk. Just the fact you can doesn't make it smart to do.