Just thought I'd weigh in here. Played several times. Finally got it after making sure that I was not in an alliance with Toyotomi (I was playing as Tokugawa).
I saved at 1614 and was in an alliance with them at it didn't work so I reloaded and broke the alliance then the event triggered. So make it to 1615 spring and then no alliance between Toyotomi and Tokugawa and I had Sanada on the enemy's side (Toyotomi). Also had Ieyasu in Owari right beside Toyotomi.