At last year's WestPac, during the all day Fazioli session, we were told that their standard for una corda set up included slightly beveling the left (bass) side of the hammers (three string unison sections), and aligning those hammers so that at full shift, the left string would not sound on a soft blow, but would on a hard blow. IOW, the bevel would keep the string from sounding on the soft blow, and on the hard blow you would get "a little extra."
Intriguing idea, and I filed it for consideration but did not experiment. This past weekend I had an experience that leads me to question whether this is a good idea.
I had an emergency call to do some prep on a Steinway D (instrument unknown to me) for a concert featuring Rach 3 with Alexandre Gavrylyuk (fabulous pianist, BTW, quite cordial and easy to work with). He was complaining about unevenness of the una corda voicing, and I had on the order of three sessions with the piano totaling 4.5 hours, with tuning included, and he would prefer to have some practice time on the instrument during those hours as well. IOW, I had to hurry.
The piano was not well prepped, though it was a reasonable instrument and powerful enough to do Rach with orchestra. It had not had una corda voicing, but also the hammers were not well aligned with the strings: at full shift, some missed the left string, some struck it full on, some struck it with the edge of the hammer (which was sharp and hadn't been beveled). I decided aligning was out of the question due to the time constraints, and that I would simply work with what I had.
Bottom line, when I met him at the beginning of my third session, he was generally satisfied, but found that about six hammers had a strange sound, sticking out when played loud at full shift. Every one of those hammers turned out to be set up like Fazioli was recommending, not making the left string sound when played softly, but it did sound on a louder blow. The objectionable sound was a nasal quality, very familiar: the sound of a badly mated hammer. (My solution was to space those hammers about 0.5 - 1.0 mm so that they missed the left string, and that solved the problem).
To add some "war story" details, we are in a somewhat rough time here in Albuquerque, with the Steinway dealer having gone under 3 years ago from the recession, no C&A Steinways available, and the orchestra having gone bankrupt. The current orchestra is trying to rise from the ashes of the old. So this instrument belongs to a patron and was borrowed for the occasion. I was called Friday evening at about 6:55 pm: can I possibly do some emergency voicing on a piano? Details are that the concert is tomorrow at 6 pm, there is a rehearsal tonight from 7 - 10, I could have access to the instrument from 7 - 9 while they rehearsed the rest of the concert. Saturday, it will be moved to the concert venue by 10 am, rehearsal 12 - 3, pre-concert lecture at 5.
I strongly advised an alternate piano - the hall's house piano, the concert Yamaha in town. The pianist wanted to talk to me, so we had a chat. He was happy with the regulation, liked the sound of the non-shift sound, was only concerned with una corda evenness. He had tried the Yamaha and would prefer this instrument if u c could be made even. He would rather not take a chance on the house piano that he would have no access to until the day of the concert (the hall had another show going on). So my "good trouper - the show must go on" and "any sacrifice for the sake of art" characteristics kicked in, and all in all it was a good, worthwhile experience.