Thanks again to Isaac Oleg for generous time taken to comment from his experience. I'm particularly interested to hear similarities and distinctions between work in Europe and work in the US. Dealing with sectional pinning issues is mostly a European challenge, although it happens to be one I have faced this week!
The extreme squaring/spacing I required of the Pleyel hammers for tonal reasons were done through shank heating, twisting/bending, and re-setting, And both of these processes are tough on the hammer centers. Some were quite loose to begin with and became very unstable by the time the work was done.
I have a method of tightening both cloth and hard bushings without additives and without disassembly, which worked with the Pleyel's through-the-butt bushings and sectional pinning. I employ the sharp point of a broach, inserting it in the bushing material, generally outside the assembled part, and turning it around the surface of an imaginary cone while applying pressure. I follow this with working the part to settle the spot I have just compressed. I can usually cut the swings of a grand hammer in half, if I want to, with stable results. My insertions are always in pairs, applying the same treatment mirror fashion to both bushings. The WNG hard bushings respond well to this treatment but require a delicate touch to not go too far. (Too far just means popping the pin, burnishing the bushings with one of their incrementally-sized burnishers, and reinserting the pin. But it's a big slow down.) Anyway, this technique worked well from the accessible inside of the sectionally-pinned bushings without taking things apart.
On another point noted in one of Isaac's comments, the picture of the treble straight-bored hammers half-filed (shown in the previous blog posting) was taken with them suspended on the support of the sliding top of a Grandwork™ Squaring Platform (SP). Because the hammers are not flared, because they have been traveled to vertical (using the SP), and because the they have been squared to vertical (again, using the SP), their being at strike is not crucial to producing an excellent job. But it is a convenient place to work on them and it is where the hammers contact and later mate with the strings. What is crucial, in my view, is that their crowns are parallel to the the plane of the action (the keybed in the piano). The Grandwork™ Regulating Rack (RR) provides the information needed for this. Then the gang filing does a superior job to hammer-by-hammer filing, particularly in achieving an overall sectional hammer shape.