Here is what that window looks like today. Actually, in person it looks slightly better; those grayish flecks on it are caulk, which must have a slightly different infrared reflectivity than the paint, because the flecks are utterly invisible to the human eye.
At any rate, I'm pretty proud of this, as it's really been teaching me a lot about masonry. At the corners of this window, there was serious plaster damage; here's an intermediate picture after some paint was already applied (I wish I'd taken better starting pictures) showing the hole. Further down is some urethane foam, my initial stopping of the myriad holes this window contained.
Now, on the south wall of this area, there's a large patch where missing plaster had been replaced by mortar. I was impressed, never having considered mortar in this application. But it works! It's ugly, but rock-hard.
So I mixed up some mortar and stuffed the holes, then the next day I cleaned out the mortar and did it again, because had I screwed up the first batch; it had become friable and sandy and could be flicked out by hand. I hadn't put enough water into the mixture, so on my second trial I was very careful: I mixed a much goopier starting mixture, carefully sprayed water onto the brick in the hole, then applied the mortar. To be extra-careful, I sprayed water onto the mortar again that evening - the longer mortar takes to dry, the better it cures (the crystallization process stops when the mortar dries, you see).
The result was incredibly solid, and adhered very well to the edges of the hole. I also applied mortar at the top of the old door frame (this window was set into a door frame), where there was about a 1/2" crack. I'd long since squirted urethane into the back of that crack to seal it, but it was still ugly.
Once I was satisfied that my second batch of mortar was going to set properly, I used plaster patch to smooth it out somewhat. It would have been a waste of time to make it too smooth, as the rest of this wall is anything but - so I just slapped some plaster onto it and painted it.
Once you gain a little physical skill working with mortar and masonry, I have to say, it is a very forgiving medium. I'm really starting to like it a lot.