Then the after, which is the other side of the door, which actually had a much larger chunk of plaster missing:
The way I've been doing this is to fill most of the void with mortar (you can see some new mortar in the hole in the top picture, but that was a little extra from my last batch), leaving just about an eighth of an inch of space. Then I put a "topcoat" on with patching plaster. On the corner I push it around to be vaguely square, then after it sets I go back with a knife and straighten it up. The result is not as strong as proper topcoat plaster, but it's easy to work with, and the mortar underneath is so strong on the bricks that it seems to work just fine. Ask me again in twenty years, I guess.
But I'm reasonably proud of the appearance. I don't really care how smooth the end result is, because the rest of the wall is so irregular that truly regular plaster would look odd. But it's a pretty strong corner and square enough. Once it's all painted I think this is a pretty good look, and it's already better mortar and plaster work than I was doing in the fall. Another couple of years of this and I'll be good at it!