This is the last chord of Francis Poulenc's Mélancolie for piano. It occurs while a piano D♭ major chord is still resonating in the bass.
How would you have the B♭♭ be perceptible, surprising, and yet still pianissimo? Any technique?
I tend to think that any single performer would judge himself negatively if this note did not sound properly, and I feel that I miss it approx. 30% of the time.
The general way to emphasize a single note in a chord is to shift the weight of your hand over the finger playing the note you want to emphasize. This can be achieved by a slight rotation of your wrist. I don't see why it wouldn't work in your situation.
Place your fingers on the chord but raise your finger over the B flat flat. As you bring your hands down to play the other notes, play the high note simultaneously. Because of the extra finger motion, you will play that note a little louder.
All you really need to do is make sure you PLAY it! Don't obsess on the 'pppp'. Play the note sufficiently decisively. Particularly on a piano with a less than perfect action. Take the weight off by lifting your arm.
I have never heard of anyone using Mark Lutton's suggestion. It seems silly to me, as it requires extra pinky work. It seems like it would be very tiring in passages with extended chordal voicing.
You can practice it by playing the whole chord an extremely light staccato and only holding the top note. After you master this, try playing the top note at different volumes while you continue to play the other voices in a very light staccato.
Horowitz suggested that when doing voicing it's helpful to think of the voiced note as "striking first" even if it strikes simultaneously.
Try playing the B♭♭ a few moments later than the rest of the chord. Similar to Lagerbaer's suggestion, you can rotate your wrist towards the top note, then play the pinky note a bit later to create emphasis—but not too late, or else it won't sound like a proper chord (≈ 0.1 seconds).
