That's true. Looks like it will have to be a Telepathy (Invisibility) roll. It says that Eldar can learn powers from Divination, Telepathy, and Fate/Battle as a Farseer or Warlock respectively.
Was hoping Conceal could be cast on a unit. Maybe Seer Councils will function like Wolf Guard in that they can break off and join a certain list of units. Sadly, they would still not be able to join War Walkers due to them being Vehicle Squadrons.