make-kpkg kernels install out of tree modules to wrong location

I tried to install out of tree modules using a custom, kernel-package built kernel. The modules are installed to the wrong directory. For make-kpkg, I used --append-to-version=-realtime-1, the kernel source directory has a localversion-rt which contains '-rt31' and I compile the kernel in the git repo. Hence the resulting version is 3.0.14-realtime-1-rt31+.

To install the modules, I use something like:
make -C "/usr/src/linux-headers-3.0.14-realtime-1-rt31+" M=$PWD \
  INSTALL_MOD_DIR="ethercat" modules_install

I would expect that this installs the oot-modules to '/lib/modules/3.0.14-realtime-1-rt31+/ethercat', but instead they are installed to '/lib/modules/3.0.14-rt31/ethercat'.

A similar problem has already been reported some time ago, and it seems it has never really been fixed inside make-kpkg: http://lists.debian.org/debian-kernel/2005/12/msg00095.html

-rt31 comes from the file localversion-rt, the rest goes into debian specific places which the kernels Makefile ignores. The kernel build uses scripts/setlocalversion to construct the version appended to 3.0.14.

For the time being, I work around this problem by NOT using --append-to-version, but defining a custom localversion-000debian which contains my own version part.