Well, I wrote this FST parser a while back: [
dump.aaronlindsay.com]
(Doesn't require DVDx or patchmii or anything... Should work on any retail disc, although there's really no error checking there)
Not sure what you mean about fopen, but there is a
libfst which lets you use fopen and such on disc files... It requires DVDx to be installed though to run it.
My example isn't that fun for opening files, since you have to use partition->Read to a buffer and give it the offset of the file. But it's an example of how the Wii disc structure works.