I was able to achieve what I wanted by copying the contents of the FlipBook node and “hacking” it. I don’t fully understand how it works, but it seems that the TexCoord node gives us some kind of structure with internal logic that isn’t a float[2]. Dividing the TexCoord by the amount of columns and rows specifies the size of the zone we want. Then adding a float to that will move the zone to the location in the texture we want. Passing this to the UVs field of the Texture Sample node “crops” the zone.
I obviously don’t understand this, but this image seems to be valid. What are the contents/format of TexCoord? Should I see it as the coordinate of one of the rasterized points?
Anyway, to crop the edges of a frame, I divided the zone into a smaller area and then added a small float to center it on in the right area on the texture.