I configured my keyboard layout to use the Generic 105-key international keyboard. I've tried AltGr (the right Alt key) plus various alt codes, to no avail. I've also tried AltGr and then " u or u ", with similarly disappointing results.
Ideally I would be able to type these characters anywhere, including text editors (all of mine support Unicode), web browsers, etc. I don't use LibreOffice, so some of the LibreOffice-specific solutions I find aren't enough.