As far as I recall, JMS had asked him to since season 1, and s5 was the when he had the time and space.

Fine! That makes sense. It was otherwise such a random time for a guest spot. Larry DiTillio's "Knives" had indeed been the last.

Anyway, it still appears crazily workoholic and mysteriously efficient to me that JMS managed to write every single episode since then (and then after Day of the Dead again) in a time when seasons were still 22 episodes per season.

It's still nuts.