Well one 'getstage WEDL08' later and my game says it's on stage 255! Is that why I've never had this encounter? Because it already happened, or at least the game thinks so? If I'm understanding it correctly, it does this by having the 'quest', The Drunken Dare, only run if the quest stage is 0. I decided to poke around 圎dit and found that the unofficial patch makes a change to the encounter to prevent it from happening more than once. I just assumed it was a very rare encounter and I was unlucky, but I started to get suspicious when I completed Namira's quest and almost immediately bumped into Sigar, a similar NPC. This is especially vexing because Immersive Weapons gives him Justice, a displayable weapon for LotD. In both my last playthrough and my current one, I've never run into this guy.