COULD BACH HAVE PLAYED AN EARLY FORTEPIANO AT THE ZIMMERMANN CAFÉ IN 1733?
According the Eva Badura-Skoda, not only could have, but "probably" did. In "Did Bach Compose 'Pianoforte Concertos'?" (2000), she lays out evidence (primarily a history of the terms used to reference the fortepiano) ...
Considering this terminological situation it appears to be a grave error
to claim that the word "cembalo" found in eighteenth-century documents
always meant a harpsichord with quills. A hammer-harpsichord might have
been meant instead, even in Johann Sebastian Bach's time. (6)
... that the "new kind of cembalo" mentioned in the Zimmermann concert announcement ...
Gracious permission having been received from His Electoral
and Royal Highness that the collegia musica may be resumed
after their interruption, a fine concert is to be presented by the
Bach Collegium Musicum tomorrow, Wednesday, June 17, at
Zimmermanns Garden on Grimmischen Stein-Weg at four
o'clock, with weekly continuation. At this occasion, a new
kind of cembalo, which has not been heard heretofore [ein
neues Clavicymbel, dergleichen allhier noch nicht gehöret
worden ], will be used. The public [Liebhaber] and specialists
[Virtuosen] are cordially invited. (6)
... could well have been an improved Silbermann, ...
Taking an unprejudiced view of the early history of the piano, the creation
of its name around 1732 and the new findings regarding the slow
acceptance of it, one is bound to conclude from this document that the
instrument Bach played on this occasion was probably the "nuovo
cembalo," namely, Silbermann's new fortepiano. (7)
... though she acknowledges another possibility (a Lautenwerk) in the accompanying footnote.
theoretically, it might also have been a "Lautenwerk," a stringed keyboard instrument,
which, if built in the form of Flügel, was likewise sometimes called "cembalo." But these
instruments were in existence since 1713. (7n8)
Badura-Skoda gives an expanded account in "The Eighteenth-Century Fortepiano Grand and Its Patrons: From Scarlatti to Beethoven" (Indiana University Press, 2017, Chapter 5, "Johann Sebastian Bach and the 'Piano et Forte'", 150 - 181).
ARE THERE ANY RECORDINGS OF BACH'S KEYBOARD CONCERTOS (WHICH HE WROTE FOR THIS INSTRUMENT) ON THE FORTEPIANO
I have not found any.
Robert Hill and Ivo Sillamaa have both recorded Bach concertos on fortepianos, but their respective instruments are either too early (K Hill after B Cristofori; thus, before 1731, the year Cristofori died) or too late (Chris Maene after Anton Walter, 1795).
There are performances on Silbermann fortepianos of other Bach works: for example, the Courante from the first Partita and the Chromatic Fantasy.