Is that a question ?
If so, the initial version will have exactly the same features as the view-manager itself - we're not doing anything fancy here.
Clement's approach is simple and sound: look up the requested view number, ask the view manager to compute the view matrix and then use that to tell the slave camera to render the view to the texture managed by the Canvas::Element - at that point, the top-level Canvas will show the requested view.
If you need functionality beyond that, I highly doubt that it would be available that "soon" - let's keep in mind how popular this feature request is, and how long-standing the proposal is - let's keep in mind, we've had various more or less complete patches and code snippets doing /some/ of this ready for years.
It's only recently that Stuart offered to help get this reviewed/committed.
Referring specifically to mirrors/mirror-textures, I don't know enough about the view manager and its built-in features, but I suppose that if an effect can be registered to be view specific, you could do all sorts of fancy things without touching any C++ code. Thorsten would be the best person to answer what's possible and what isn't.
However, speaking in general, you could have really fancy FLIR/night vision displays, including displays showing different view-points in a PIP (picture-in-picture) fashion - none of this would require any coding capabilities, it's really as simple as doing some XML copy&paste to add a Canvas enabled display to your cockpit and tell it the view to be rendered.
Thorsten also mentioned that he was interested in using such a Canvas view for the shuttle RMS arm.
Other than that, the use-case does not matter - i.e. this would also be of interest to people interested in non-civilian flight: