My note about synthesised expression has generated some interest on-
and off-list so here's more about it.

The idea isn't mine -- it's been around for over 100 years and I first
encountered it in a Hupfeld cafe piano which is described in MMD at
https://www.mmdigest.com/Archives/Digests/200101/2001.01.12.11.html 

This was a basic 88-note instrument with a single conventional
regulator for the stack.  The only unusual feature was that the
regulator had an abnormally weak spring which added a positive slope
to the 'Flow vs. Demand' characteristic.  As a result, the more notes
playing the louder it played.  This has little musical credibility but
the effect of doing so, as demonstrated by the file of "Silvery Moon",
is actually quite pleasing.

The software to emulate the effect could hardly be simpler -- it merely
counts the number of notes playing at any given instant and adjusts the
velocity proportionately.  I wrote the original program in 1998 as a
throwaway experiment and it's very primitive.  It's also in Quick Basic
which won't run on modern PCs.  A properly written utility would have
the potential to provide us all with a vast stock of 'improved' MIDI
files of standard rolls and I'm hoping someone with programming ability
will offer to tackle this.  I'll gladly provide full co-operation.

Regards,
Richard Stibbons