Funny that you use the word 'fortepiano'...
I pulled the trigger on the Hogwood edition of the first set (Op. 5, I think). I'll let you know after I get them.
Prometheus,

usually "fortepiano" is the word for the instruments from Cristofori's until, I would say, the Broadwood Beethoven owned (you could argue a lot about the early Bosendorfer's, but it is unimportant for JC Bach).
J.C. Bach worked in England, although I am unsure about the chronology of when his keyboard sonatas were composed. I tend to think he was more familiar with the Clementi and Broadwood pianos, rather than, say, a Walter or Silberman like the ones Mozart played the most. I tend to think Bach's style is much closer to Mozart than to Clementi, though, so who knows.
In any case, Bach sonatas were written for an instrument materially different from a piano (as we use the word today).
[late edit: JC bach played concerts in London on a Zumpe square piano. I know very little about these instrument, but will let you know once I learn more and how it may be reflected in the writing of the sonatas].
Cheers!