Have updated to Studio One 5 and Spectralayers 7 Pro. Here's info from Steinberg:
Starting with Studio One 4.1, you can assign SpectraLayers as an ARA plugin to your regions.
- Go to the Effects tab on the right side of the screen.
- Hold the ALT key, and drag SpectraLayers onto an audio region.
If your DAW is ARA2-capable, you can use SpectraLayers as an ARA plugin. This allow a very tight integration between the host and the plugin : every region you send to SpectraLayers appears as a layer and remains synchronized with the host. If you move it or resize it in the host, the corresponding layer in SpectraLayers will be moved and resized as well. Every audio modification you do in SpectraLayers is automatically transferred to the host during playback or rendering, and saved with your host project.
The ARA protocol requires a 1:1 synchronization between the regions in the plugins and the regions in the host. For this reason, you cannot create or destroy layers within SpectraLayers in ARA mode. If you need more layers in SpectraLayers, your need to send more regions from your host to the plugin.