Hi,
As you have found, there is unfortunately no raster symbology feature in the current WPF API. In the future, for the new .NET SDK, we will be adding direct read of the rasters and I would expect to see some rendering options exposed at that point.
In the existing WPF SDK your raster symbology would need to be defined in the GP model/script you are executing, and then rendered on the LocalServer. ArcPy.Mapping is *not* currently supported in the LocalServer, therefore unfortunately RasterClassifiedSymbology is not available. However, I believe it should be possible to set a default renderer within the model/script parameter?
Cheers
Mike