sorry to bump an old thread but when downloading from one particular newznab indexer (I wont name them here publicly) who uses a custom obfuscation method, the par2 files are each renamed differently - Alt.Binz cannot seem to recognise that they all are mislabeled par2 volumes as they have different names, and unless you rename the par2 volumes manually you cannot use them to par2 repair.
Then, the other related issue is that if the file doesn't need repair, and since I have par2 volumes up to 12KB set to automatically download whether I need them or not (saves time since Alt.Binz doesn't need to check all the files twice) is that these obfuscated par2 volumes, which are different to the original .par2 index file, don't get cleaned/deleted after a successful unrar in the cleanup.
I can provide examples via pm if required.