I've gotten this working now. Thanks for the guide! But I thought I'd share a weird experience I had when I was doing this,
After I managed to export the rasterization .tga file, it was very peculiarly upside down. With the resulting satellite background image being south at the top, whilst the map geometry data had north at the top, as expected.
Fixing this was as simple as loading the file in an image editor, and flipping it the right way. I used Paint.NET with an extension to accept TGA files.
At first, doing this essentially corrupted the file in the eyes of the Workbench, but I checked the export options I was using, and I turned off RLE compression, which solved the problem.