ÁÉÍÓÚ
*"...On OSX, LibreOffice does not integrate with the OSX method of entering accented characters (where you can hold down a character longer and a selection pops up)..."
Try hitting:
* ALT+e followed by the vowel you want, to add an: áéíóú
* ALT+i followed by the letter you want, to add an: âêîôû
* ALT+u followed by the letter you want, to ad äëïöü
* ALT+n followed by n for an: ñ
* ALT+c to produce ç
* ALT+o to produce ø
[or, alternatively, switch your keyboard to use Irish layout and ALT+any vowel will type that vowel with an áéíóú, which saves an extra keystroke in languages which only use that type of accented character].