There is a layout which is the major (main?) PITA for me, all the time since I started dealing with XKB in GNOME. No, not the Hungarian one. The Canadian one. Historically (which means good old xfree 4.2 days and earlier) it contained two groups (4 shift levels each). They were split into two variants, so people had to combine them. In latest xkeyboard-config releases, I tried to compile single variant, with one group and (drums...) 8(!!!) shift levels. And I found there were not special XKB symbols defined in keysymdef.h for switching to the levels 5 and above. I tried to mimic them using rarely used functional keys (F21) - but it seems it was not good enough - still, toolkits/apps interpreted these F* keys as valid input. So, I asked xorg to add several more characters to keysymdef.h: XK_ISO_Level5_*. A couple of days ago, Daniel Stone committed them to CVS (thanks indeed, Daniel).
Now, I am kindly asking all people maintaining Xorg builds in various distributions to apply this patch when releasing their coming updates. It would not break any API/ABI. It would allow me to commit the normal fix for the Canadian layout to xkeyboard-config, so people could use it.
For more details, see the fd.o bug.