Wikipedia lists the NA retail debut at May 11, 1995. There's nothing about a debut date for the add-on so I'm assuming it was a launch feature.
Given this, I'd still bet money on my first guess that the Pluto was a NA Saturn prototype scrapped cause it was too expensive and Professional Jerk Stolar(XD) wanted to cycle the Saturn out.
You guys probably already knew this, but the North American Netlink doesn't require servers. Maybe the Pluto was an experiment to streamline netplay...
EDIT: Oh wait. Here's a news story about SEGA debuting the thing:
http://news.cnet.com/Sega-catapults-to-the-Net/2100-1023_3-239291.htmlI don't think it changes much though.