Hi Sean,
We’re aiming to support these kind of use cases (though we’re not sure yet to what extent in the initial release). We know that many users want to be able to plug in their own combinations of VSTs, and they want to use ‘arco’ samples from this library and ‘pizz’ from another. So what you will be able to do is configure the rack with all the VSTs you want to use, load your preferred presets, and then tell Dorico the Expression Maps or ‘Playing Technique’ for each channel of each VST. We call each destination an ‘Endpoint’ (basically the plugin + channel + expression map).
You can then map each of your source Instruments to the Endpoints you want to use. So you could load an arco patch in one slot and a pizz in another, then in the Instruments control you just point it at these two endpoints. When we come across the pizz in the score we’ll switch playback to the other endpoint. In the score you will just have one source stave, but playback can be split across many endpoints.
I’m not sure at the moment how best to support switching to another sample library just for a different section. It might be a matter of making a custom playing technique for it. The same might be true for the multiple spiccato samples.
You should be able to have per-voice playback routing, so that should give you a way achieve the ensemble/solo effect, or that could probably be also done with a playing technique change.