This would actually make the crystal ball worth bringing! 
Looking forward to your final result of this debate
The crystal ball is ALWAYS worth taking these days. Unless of course its open list...
However, if you would use this trick to force the assassin out just because ppl come to that consensus then you prolly wouldn't mind using the crystal ball to measure distances. It's perfectly leagal and you can do it. Put him with a guess weapon. The ball says you can use it at literally "any time" as opposed to the begging of a phase or turn or something. That means that you can "search for secrets" right before the cannon the crystal ball is with fires. Make sure you measure though so you aren't cheating your opponent by asking for units that are out of range...
You can even ask during a specific unit's move. For example, a unit of shades starts its move outside of the 24" bubble, they move their 10" and end their move outside of the 24" bubble. BUT during the move they barely pass within the 24" bubble, you can halt their move, and ask what is in the unit. Turns out there's an assassin there! Hey, maybe you'll even get to force him to be revealed!
This is all against the spirit of the game of course, but it is completely legal. And "revealing" the assassin with the ball is against the spirit also, but maybe it will be legal. What I'm saying is that if you like the idea of forcing an assassin out with RAW, you'll love the auto-measure anything within 24" application of the GPS satalite...erm I mean Crystal Ball...