Comment 19 for bug 7848

I do not have Mac hardware, and so never played with XKB configuration in this area.
Anyway here are some hints to help debugging:
 * XFree86.0.log contains messages displayed when X is launched; when XKB configuration is changed in GNOME, messages are writtent into GDM log file, so providing these error messages is very helpful.
 * setxkbmap has a -print option which displays what will be passed to xkbcomp. When patching XKB files, running
   setxkbmap [options] -print | xkbcomp -w0 -I/etc/X11/xkb - :0
is handy, or the output of setxkbmap is stored into a file which is edited and passed to xkbcomp.
And here is an example with macintosh layout: compare
  setxkbmap -model macintosh -layout us -print
and
  setxkbmap -model macintosh -layout us,gb -print
It seems very buggy to me, so I would not be surprised if macintosh could not handle multiple layouts. And GNOME keyboard switcher should only display symbol files which are relevant to the model, in this case files under /etc/X11/xkb/symbols/macintosh/.

About the Hungarian layout, I sent a file to http://bugs.debian.org/269929 and am interested in knowing if it works fine.