You can change the font of the text used in chord names (Fonts under
the View Menu), but a chord font wouldn't work the way you want. For
example, Em7b5 would add 5 chord diagrams...one for each character in
the chord name. Unfortunately, you won't be able to add chord diagrams
in place of the chord names in the way you want. You can only have them
listed at the top of the score.