Here's a naive thought that might just provide or inspire an intermediate workaround. (I find software specification is so much easier when done in complete ignorance and without troubling to consider the entire system, anybody else's requirements or platforms other than one's own

)...
- There's already the facility to load and simultaneously render two different scenes, used for timeline crossfade.
- There's already a separate Spout output with independent resolution. Suppose it had an option of an independent source too - the preview scene.
- The Spout output could drive a preview window external to Magic (perhaps a display of a virtual webcam?). Nothing tricky needed for that bit.
- Alternatively, the "main" display could be the preview leaving Spout to feed downstream processes.
- Perhaps use similar scheme to Resolume Avenue for selecting the scene to be previewed: Click on the playlist scene number for (instant) transition. Click on the playlist scene name for a preview.
Ok, one might have to sacrifice fancy transitions between scenes, but for live performance I'd rather have an instant cut which can be exactly on the beat.