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
This plugin may break in a future version of Opus if we remove the non-Unicode API.
If you find the plugin isn't working, try this alternative one which can handle the same format (although I don't know if it is the same, or better or worse at this particular format) and uses the Unicode API:
The link at the bottom of the root post here works for me. The plugin won't work with 12.10 & above unless updated though (or unless we add back the ANSI API; we're waiting to see if there's feedback about removing it, as we think only two plugins were using it, and both have alternatives).
I've added a local cache for the file as well, in the root post.