If I want to transcribe/arrange a harp piece for the piano, and the piece contains lots of bisbigliando passages, what would be the best way to imitate that charming effect on the piano, and how would I notate it as clearly as possible?
The best way would be, well, take a grab in the piano and play an actual bisbigliando on the strings! But that's hardly practical.
Short of that, I don't think there's much better you can do than using both the sustain and una corda pedals and then using a regular pianissimo trill.
If you have the option to prepare the piano, or use a digital one, there are a couple of options that could get you closer. Soft cotton wool on the hammers and/or strings, echo devices for a denser sound, or detuning the next key down a semitone so you can use two keys for the same note (again facilitating denser tremolo) might be things to consider.