Not many FTPds nor many FTP clients support SRP, however it doesn't look hard to implement, and should be nice to have as an option, taking into account ioFTPD's modular design.
http://srp.stanford.edu/ discusses and provides sample code for implementation.
Again, this is not really needed, however it's much faster and secure than most other implementations of an auth method.
More secure than challenge/ response authentication, it doesn't matter if you know the hash.