If you only need to have you last chord playing, perhaps you could simply use a regular sustain (CC64) together with the Plugin Persist Scriptlet:
With this Scriptlet the notes which are playing go on playing until you stop to play them, new notes are filtered out, and the connected plugin is definitely bypassed once you the last played notes is released.
Would this work for you, or do you still need to play rhythmic chords using the sostenuto notes?