Do you use the "cover_level_icon_postfix" field?
That feature adds only a postfix to the default icon name (Icons_cover_heavy). So when you set the value ModIcons_a123456789, the game will search for Icons_cover_heavyModIcons_a123456789.
Therefor I think it is not possible to use custom icons for cover.
I'm assuming you mean cover_level_icon_suffix and yes that is what I tried. Seems to be not possible if it's doing a lookup with a specific string.
Thanks anyway