sigh
...so, I was just screwing around with plug-ins, and I made a mission with a TravelStel of Heraan (360). I tried it with my Auroran string pilot (for whom Heraan's visible instance is 431), and Nova did what it was supposed to, and allowed the visible version of Heraan to act as the TravelStel. However, I also had some SpecialShips (10 large Auroran warships, kinda hard to miss) set to appear in the TravelStel system.
They didn't appear.
In short, if you set a TravelStel (or ReturnStel, presumably) to a spob that is invisible but has a visible instance available, the engine will let you use the visible instance, but SpecialShips set to appear in the TravelStel system (that is, -3, not the actual ID of the system) don't seem to appear in the visible instance of the system.
Can anyone confirm this behavior?