How to run a plugin against a submix of sends

Question

I have an effect plugin hosted in an auxiliary bus.  While it successfully processes all sends routed to that bus, it does not seem to be executed against a submix of those sends (rather, it sounds like it's processing each send buffer independently).  I can give more detail if needed, but the main point is that I’d like to have it run in a context equivalent to the “submix voice with global reverb” as shown in this picture -- once per Wwise's thread tick, against the submix of a set of sends:

Comments

More context:  I have an effect which needs to respond to game state, approximately every 30ms.  It has long tails, on the order of seconds.  What I was expecting was for the sound to play and for the parameters to modify the effect’s behaviour as it processed through the tail.  However, it sounds like the parameters are only applied on the start of the sound and aren’t modified as the tail gets processed.  Furthermore, other sounds -- going to the same bus at different times -- do get the right initial behaviour, but also fail during tail processing the same way.  One can hear different sounds playing with different parameter settings which should not be possible if the sounds were mixed together before being sent to the effect.