I've noticed throughout the game, and on several custom maps, that players have the ability to shoot a portal on a panel and it will be automatically positioned and oriented a certain way. I tried looking through the documentation for this, but with no such luck (probably because I have no idea what this feature is called!)

Anyone have any ideas?

The entity you are looking for is info_placement_helper. You will need to make sure the entity is "facing" the direction that you want the portal to be placed it. It needs to be flush against the surface, otherwise it won't snap the portal placement there.

That did the trick. Thanks!