I wonder if fontdiff [1] might help compare renders. The existing tool would not be usable for this bug, as it compares two fonts (probably two different versions of the same font is the intended usage). But it does the comparison using two generated image files. So if you have two images (one from the old installer, one from the new installer) maybe there is code in this tool that you could expose to the command line to compare the already generated images.
[1] https://github.com/googlei18n/fontdiff -- Bobby de Vos /bobby_de...@sil.org/