well, after some research with google, I found that you can press the "Compose" key and then "~" followed by "u" to generate "ü". The Sun keyboard actually contains a Compose key, but the Z keyboard does not, so I mapped the left japanese key (keycode 97) to be the Compose key. however, after reloading the modified keymap, and pressing the newly mapped Compose key with the corresponding keys does not produce the desired effect. It appears the Compose key is ignored despite it being mapped and the keymap containing many compose mappings. none of them work. anyone know how to get them working? I tried this on opie without success. the deadkeys (ie using the compose key) does not work. I will try GPE next where the deadkeys needs to be mapped to "Multi_key" in xmodmaprc instead of "Compose"....
So far, I still think that Sharp ROM, despite its hacked 2.4.x kernel, still provides the best usability experience, ie keyhelper works. I wish there was a keyhelper equivalent in OPIE. keymaps and xmodmap documentation is not very helpful.