The same style which I posted in does not work under 2.10.0 any more. Within the <ChannelSelection> I can permute values for <SourceChannelName> freely without having an effect on the result. Also my VendorOptions are not interpreted at all. GeoServer's log does not print any errors and the server returns a regular image, as if rendered with the default raster-style.
Here is the style from GEOS-7235:
Ubuntu 16.04 Server x64, Oracle Java 1.8, Native JAI