Richard, as I'm sure you're aware, this whole maze of license compatibility can initially be quite confusing. What is the nature of the incompatibility between MPL and GPL?
The GNU GPL is a true copyleft: it says that modified versions of the program must be distributed *as a whole* under the GPL. Therefore, to add another file to a modified version of the program, the criterion is this: do you have permission to include that text in something covered by the GNU GPL?
The MPL is incompatible with the GPL because it does not give permission for this. The LGPL explicitly gives permission for this, and that's how it manages to be compatible with the GPL.
Any real copyleft would raise the same issue as the GPL.