Abu Simbel and Courtesan's Guild
The Courtesan token stays on a Leader that gets entombed. This is based on Eddie the Cranky Gamer’s post on BoardGameGeek (to which @Jean-Sébastien links to in his answer, too), who says the he’s asked Repos:


  I put the question to Repos and the answer was that the Courtesan stays in play, with the same assumed power of the now-entombed leader. 


Putting the Courtesan token on an already entombed Leader is not possible. At least that’s how I understand Ovidiu Cirlugea’s BoardGameGeek post, quoting the game author Antoine Bauza:


  You cannot copy it once its buried […]


Putting the Courtesan token on a Leader which is entombed on the same turn is possible, as detailed in that same BGG thread:


  Yes, you can copy a leader, even if your neighbor choose to bury it during the same turn. The copy remains, even if the original is buried after !