Render subsection, produce full resolution image Background

In an animation, text scrolls horizontally across the screen (from left to right), stopping on the far right. For example:



The background is transparent and no other objects are in the scene. Blender creates PNG frames at 720p, as expected.

Problem

Blender spends time rendering image parts that will only be transparent. The CTRL+b command allows for rendering a subsection of the image, but appears to produce an image that is not 720p.

Question

How would you create a 1280x720 PNG image but only render a horizontal band (see below) where the text is animated?



Related


Render just part of a final image from Cycles?
By default, using the render border (CtrlB) will still produce a full image according to the set render resolution (I think this what you want).

The image will only be cropped to the border region if Crop is enabled in Render settings > Dimensions:



With Crop disabled, only the border region will be rendered, and the rest of the image will be filled with transparency.