I tried using the config file. Rendering failed with the log saying:
[Fatal] Rules or art config file could not be loaded! You cannot render a YR/FS map without the expansion installed
[Fatal] Ini files couldn't be loaded
[Error] Could not successfully load this map. Try specifying the engine type manually.
Rules loading failed. Please make sure the mix directory is correct and that the required expansion packs are installed if they are required for the map you want to render.
It appears an error ocurred during image rendering.
I managed to make it work by adding the Mental Omega directory directly to the config file. Without it, the program failed to find the necessary files despite having the correct folder in the second dialog.
(Also managed to find out that I can't render voxels at all, modded or original, but I'm quite sure that's unrelated)