Rallentando and accelerando in MIDI

[David Sumbler]

Rallentando and accelerando are supported in MIDI output if the
articulate.ly script is used.  Looking at the script, I can see how I
can additional synonyms for rall., rit. etc. that the script would then
recognize.  I can also begin to see how to change the amount of rall, or
accel. by modifying the script.

What I should like to be able to do sometimes is to alter the rate of
rall. or accel. "on the fly" - i.e. during the course of a piece.  I
suspect that this might be possible using some Scheme statements, but my
understanding of Scheme is insufficient for me even to know if this is
possible.

Can anyone tell me how to do this, or if it is even possible?

David