This question comes up every now and then and it doesn't seem to ever get a good answer. I'm not sure that there is anyone out there who has systematically crossed all the lines to see. I suppose they are worried that some might be incompatible so mainly stay within lines.
One of the original sources of information about incompatible ghost lines goes back to a "ghost" that VPI had that was described as having a different feel to it's scales. I'm not sure if that line actually proved as I haven't heard about it since but I understand that it wasn't compatible with what we now call ghost ball pythons even though I think it was the original source of the name.
My suspicion is that all of the currently available proven ghost lines might well be compatible and that the differences in color might mainly be due to other genes in the lines (just like normals vary in color). This is just my theory and not tested. As part of this theory I think that the other cases of incompatibility you hear about involve imported ghost looking animals that might not be actual genetic ghosts. Perhaps there is some non-genetic condition that can cause a ghost looking animal. If anyone has bred two proven genetic ghost lines together and not produced ghosts I would like to hear about it and then I can forget about this theory.