Hey here's something I was wondering while working on a patch today using the Uni-Scream library. Wonder if there would be a way to have a negative key switch. In other words, in my head I've always considered a key press as a positive event, not sure how the engineers think of it probably more like binaries, 0 or 1's but, what if when releasing a note it could trigger a layer that a certain sound I want to use is on like raise the key to an explosion or something like that. So one sound with the key press then another when the note is lifted. I know that tons of older Roland gear used a feature like that when doing orchestral sounds.
This is certainly possible. Many samplers use what are called "release samples". One tricky aspect is that such "note-off sounds" can only ever be triggered, as there is no notion of the note being held down and eventually released.
John and I have discussed the idea of a "trigger on release" mode for Guru Sampler, and this is already on the to-do list.