I’m slightly disappointed by Lion’s iOS inspired feature to show a selection of accented characters on key hold.

On a German keyboard, when pressing a, I am able to choose between ä, â, à, á, æ, ã, å, ā. So far so nice.

However, I feel some characters are missing. There is no ç (although I could type this one using option+c), no ğ and no ł either. On the other hand the letter ń is shown (which like ł is more or less only used in Polish) but to which OS X does not show an upper case version. shift+n yields no results.

Is there some way to enhance and expand this set of letters with own additions?