There is no way to dither run-lenght encoded TGA images on the fly. Dithering them would require decoding the whole image into an intermediary buffer first, and this would temporarily increase the memory consumption by a significant amount.
Maybe later, but not now. This means that uncompressed TGAs will be dithered, and that run-lenght encoded TGAs won’t.
By the way, the non-dithered borders mentioned in the previous post have been fixed.