You might also try AllChars or something similar. It allows you to press a particular hotkey and then two more keys to generate an accented (or otherwise not-on-your-keyboard) character. For example, if you press the hotkey, then 'n', then '~', it will generate 'ñ'. It has some limitations, but it should do what you want, and the keystrokes are a little easier to remember than ALT-0223 or whatever.