Wow... I now see what you were talking about. I just looked at the mml. It took me about 20 minutes of study of one line alone but I eventually figured out what is going on. It indeed uses gain fades which I had completely forgotten about, until I figured out how they work and then vaguely remembered reading about them on some article about the SPC. I'll explain for the benefit of those who, like me, didn't see it.
Basically when you set gain,, either with $ed $80 or in an instrument, the gain value can have several meanings depending on what range it's in. If the gain is set from $00 to $7f it is treated as a direct gain value i.e. $00 is silent, $7f is full. If it is set from $80 to $ff a gain fade is applied, which fades the current gain to full or silence, depending on whether you are using an increase or decrease fade.
Going from $80 to $9f uses the first decrease fade type, not sure the shapes of the fades right now, but it seems to be the most abrupt of the two. As you probably guessed, the higher the number, the faster the fade. Starting at $a0 switches to the other decrease type which is more gradual. Its speed goes up to $bf. The pattern continues for the increase fade types. I hope this make-shift explanation makes some sort of sense.
To illustrate how a sforzando might work, I could do the following. This isn't optimized, it just shows how you might set it up. Remote codes would make this more practical.
#0 l4 @4
$ed $19 $20 c ;initial accent and diminuendo
$ed $80 $c7 ^ ;gain fade for the swell
$ed $80 $bb ^2 ;another gain fade for release, though straight-up ADSR probably could've worked too
Hope this helps somebody.
Make more of less, that way you won't make less of more!