Problems of the GUI not displaying certain characters, even though a suitable custom font is installed, have been mentioned several times. Lately, I had this problem myself. I installed a Sanskrit-English dictionary (Sanskrit being transliterated with diacritics). In order to be able to search the dictionary, I patched the keyboard. But some characters did not show (cf. screen_01.png). They did also not show in the list of suggestions (cf. screen_01.png, screen_02.png). The problem is that the automatic font selection does not always yield the wished for results. What I tried was to remove the font or the fonts that get applied instead of the font that should be applied. Actually, I renamed the directory that contains the on-board fonts.
One result of this is that at start, nickel does not have access to any fonts (cf. screen_03.png), but as soon as one opens a book, nickel uses the installed custom fonts (cf. screen_04.png, screen_05.png).
Code:
mv usr/local/Trolltech/QtEmbedded-4.6.2-arm/lib/fonts usr/local/Trolltech/QtEmbedded-4.6.2-arm/lib/fontsx