I was trying to crack the custom format Rovio/Housemarque/Funlabs used for textures, unfortunately I didn't crack the texture files entirely yet but I tried my best for their header.
ABTrilogy Raw Texture MIP header (20bytes [Wii], 1C bytes [Wii U]):
4bytes: 4D495068 (MIPh [MIP Header?])
4bytes: Bytesize of texture data [20 bytes AFTER the GCT header/160 AFTER the WUT header] Big Endian) 00 replaces bytes in small files
2bytes: Unknown value 1
2bytes: Unknown value 2, same as unknown value 1
2bytes: 40 (Wii), C0, (Wii U), texture offset
2bytes: Texture width
2bytes: Texture height
2bytes: Unknown (Only present on Wii, used in big files, filesize?)
2/4bytes: 0, end of header (Wii uses 4, U uses 2)
If you want samples, check viewtopic.php?f=7&t=13658
for Wii and viewtopic.php?f=7&t=12605
for Wii U.