Thanks koan for the answer.
Yes, I am using a Greek font. In particular, I installed two Unicode TrueType fonts (Consolas for fixed font and Tahoma for system font) and used the included TrueType manager to install it (I've also selected the correct codepage ISO-8859-7 for Greek).
No, I am not seeing accented characters, just latin ones. For example, after selecting the Greek keymap and trying to type the Greek leter 'alpha' (A on keyboard), I am not getting an accented character but a simple plain 'a'.
I've also installed sbar_keymap v20.6 (http://tokai.binaryriot.org/public) and using it's character palette I can see the Greek characters after the latin ones (which means that my font does support Greek), but even if the keymap is swapped fine, I can't write in Greek.
So far my MacMini MorphOS experience (since Tuesday) has been great but this issue is spoiling it because it is essential for me to write in Greek.
Again, many thanks in advance.
P.S: Is there any documentation available on how someone (with expert knowledge of C) can create a new MorphOS keymap? I suspect that the included 'pc104_gr' keymap does not either support Unicode or Greek keyboards at all.
(MorphOS 2.4 - Mac Mini G4)