Sure! I hope this gets into use, as I could see some neat applications for this.
First off, let me note that this isn't using AI, its scripted. However, it really is using portals, and not duplicating NPC's or anything silly like that.
The way I have it set up is using path_corners. I place whatever path I want in front of a portal, then one directly behind it. (Note: so far the path_corner going behind the portal must be in walkable space, or the NPC won't go to it. A workaround might be to make a brush that is walkable only to nonplayers somehow, or to place the portal a few feet from the wall like I did).
Then place a path_corner in front of the other portal, followed by whatever path you want the NPC to walk afterwards. Then place a trigger_multiple in front of the second portal, filtered to only the NPC you want. This trigger will tell the NPC to stop walking to the old path_corner that is behind the first portal, and tell him to start on a new path in front of the second. The trigger outputs to a aiscripted_schedule with the schedule "Walk Goal Path" and the goal entity being the path_corner in front of the second portal.
Make sure you have a filter_activator_name set for the trigger, so it knows to only do this if the NPC walks through. Set this up like you would for using the box to trigger a button.
I am posting a youtube video of this right now, and will post the URL when its up.