I had given that some thought already, and while there are a number of other things that I want to do first, the idea I've been kicking around in my head for doing something like that in Paragon Chat goes something like this:
Each player gets 1 (one) 'companion' NPC they can summon at a time. Summoning a different one dismisses the first. These would be selectable from a number of choices including vanity pets, veteran pets, buff pets, mastermind pets, lore pets, and a few others.
The AI for these pets would be very barebones, just enough to make a beeline to you. If they got stuck somewhere, they'd probably be unable to navigate around complex geometry, so they'd have a fairly short tether range to just instantly teleport to you if they get too far.
Companion representation over XMPP would be very lightweight. It would basically be just 'summon' and 'dismiss' events -- no position sync at all. Keeping things somewhat synchronized would rely on each instance of Paragon Chat running an identical AI and feeding it a similar target position of the player to follow. Obviously it wouldn't be exact, but should be close enough for RP purposes without taking up a whole bunch of extra traffic.
A stretch goal would be to allow for sending emote commands for companions, or at least ones with sequencers similar enough to player ones to handle standard emotes.