Didn't really find anything on site about creating custom brushes for texture painting. I eventually did figure it out.
If anyone needs a reference:
-Under the brush tools, create a new texture, set to View Plane.
-In the properties panel, under texture, select the brush texture and load a PNG with transparency.
However, for some reason this results in the brush only painting black, which isn't exactly useful. Anyone know how to get it to do other colors?