when it's
tail is done. Once it is silent, it does not need to be
processed._
this sounds very dangerous to me.
your description sounds OK. but ... my kawai k5000 is an additive
synth with a moderately interesting FX bank. its still producing sound
for most patches long after all of its "voices" are off. if someone
were to write a plugin modelled on this kind of design, you can't
really know when the top of the chain is silent.
The top-of-chain plugin (synth) will tell you when it is silent, of course!
I think it can be a massive, coarse-grain optimization, if we can make it
work.