In the very rare case that you will ever need to do such a thing, let’s dive in a take a look at the steps you would take to actually make a “good” 4-way close harmonization of a melody that exhibits an appoggiatura over a diminished chord.
In the example above, we see that the Bb is an appoggiatura. It is a dissonance on a strong beat caused by a leap in one direction, and resolved in the opposite.
In four-part writing harmonization, the appoggiatura over the diminished chord will replace the next lowest chord member. It that case, the Bb‘s nearest close chord member from below is the Ab:
Also, take notice that it is a whole-step away, too. Next, you would harmonize using the tones of the Ab diminished chord, but omitting the Ab pitch.
From there, you can harmonize the rest of your passage in 4-way close, drop voicing, etc.
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