I am not an expert on expression coding but I have done a fair
amount of thinking about this topic and I tend to agree with
Bob Taylor's assessment.  The Welte system uses only two speeds
of crescendo/decrescendo.

It seems completely impossible that any system, whether based on
pneumatic or electrical logic, could convert note intensities to
expression coding in such a short period of time that a roll with
expression could be produced in an hour.

Even the analysis of a simple crescendo and conversion to
appropriately timed codes using a series of note intensities would
be difficult (more likely impossible) with technology of the day.
More difficult encoding would be completely impossible.

Modern computers could probably do a passable job but a good job
would likely require additional hand editing.  It is conceivable
that note intensity could be recorded by a variety of methods and
this information used together with sheet music and observation
during the performance could be used in the editing process.

Spencer Chase