It only works for me if I specify two -I, the first one blank. My guess is that the first blank -I clears the default path. The man page for setxkbmap (on Debian Lenny) doesn't mention anything about this:

-I directory
Adds a directory to the list of directories to be used to
search for specified layout or rules files.

But the man page for xkbcomp (on Debian Lenny) says:

-Idir Specifies top-level directories to be searched for files
included by the keymap description. After all directories
specified by -I options have been searched, the current direc-
tory and finally, the default xkb directory (usually
/usr/lib/X11/xkb) will be searched.

To prevent the current and default directories from being
searched, use the -I option alone (i.e. without a directory),
before any -I options that specify the directories you do want
searched.

It may help figuring out what is going on if you can report what the variations on the setxkbmap command are and what variations on the error result.