Changing the keyboard settings to French does the trick. Generally, for French accents I use the US International keyboard, which does not allow insertion of accents in WA.
Try adding a French keyboard and switch to it using the Shift-Alt combination (you probably use it to switch between Greek and English) or select it in the Language toolbar.
Can't tell about Greek characters, but I succeeded with Romanian diacritics very nicely.
[Edited at 2008-10-18 09:51]
I did try with French keyboard. It only works with characters which do not need a dead key (i.e. one accent key and one letter key).
And of course, Greek keyboard is set by default when testing; so, that is not the case.