opening.bnr is a file related to Wii homebrew development. The file is one of the first read by the Wii when a new DVD is inserted, and it is used to display the banner (the channel animation). Even though it shares the name with the file on the GameCube, the content of the file has no resemblence to the GC opening.bnr file.
opening.bnr consists of two parts, a header which is (almost) identical to the latter of part A (starting from Zeroes_1) in content.bin, see Content.bin file structure. However, the last 16 bytes contains some randomly-looking bytes, most likely an MD5 sum.
After the header follows an U8 archive. This is an archive consisting of the following structure:
/meta /meta/banner.bin /meta/icon.bin /meta/sound.bin
The banner.bin and icon.bin files are LZ77 compressed U8 archives themselves, each with an IMD5 header, containing what looks like an MD5 of the file (though this has not yet been confirmed). sound.bin has an IMD5 header, but is neither LZ77 compressed nor an U8 archive; instead it is of the type "BNS".
More information on LZ77 compression can be found on Wikipedia.