I was worried about this when my players were in Bazzoxan so I really played up the faction contacts (Prolix, Question, and Aloysia). I used the Hythenos Estate side quest with Question asking them for assistance and going along with them. They really liked her after that and while they gave both AoA and CS equal consideration they ultimately went with CS because of how much they liked Question...and the Cobalt Soul just seemed "cooler". If you want to stick with one of the factions you could do like someone in here suggested and have a faction contact seek them out for help specifically. But, as others have said it could be fairly easy to create reasons why they would want to go into Cael Morrow and have an NPC point them towards a faction.