long time due to the forum failure. In the meantime, I asked at the Xdotool mailing list about weird characters, and here's the reply by the developer himself:
Yeah some of the character mapping names can be confusing. In general, 'xdotool key' characters valid are any 'key symbol' or XKeysym.
The 'keysymdef.h' file in X11 will help you. Any 'XK_'-prefixed name is valid; simply drop the 'XK_' part.
http://cgit.freedesktop.org/xorg/proto/ ... eysymdef.h
For example, to have xdotool send 'a' with umlaut/diaeresis, do:
xdotool key adiaeresis
The capital letter form is 'Adiaeresis' (look in the above keysymdef.h for XK_Adiaeresis)