@brad
I think there may be an error in the implementation.
Insert the Sostenuto filter.
Play a note and then hold down the sostenuto controller.
Lift up your finger, the note continues, which is correct behavior.
Place your finger back on the note and release the sostenuto.
The note is cut off.
This is incorrect behavior.
For comparison, the PIZ MIDI Sostenuto plugin allows the note on to be reintroduced and does not cut off the note when the sostenuto is released.