I have written a viewer plugin using the FreeImage Lib (http://freeimage.sourceforge.net). <-- Note: You don't have to download FreeImage itself and the FreeImage.dll won't do anything within Opus. See below for the plugin Zip.
Currently it is just a very simple viewer. Only the major formats (DXT1, DXT3, DXT5 and the RGB(A) formats) are supported; palette formats and more esoteric formats like floating point and v8u8 are ignored. Neither Mipmaps nor Alpha Channels can be displayed and it can't be configured. But at least it's better than nothing
It can be downloaded at http://www.phoenixgrafik.de/coding/dopusdds.zip