rather than using the array.array for your three byte header with manual
parsing, please use struct.unpack.

Whats a good way to test that this works?  It'd be nice to have a unit
test (a test_ftplib_net.py perhaps?) though I realize ftplib currently
has poor test coverage with no such explicit network test existing
beyond things the urllib tests might do.

also, could you make the appropriate documentation updates against
python trunk's Doc/lib/ftplib.rst.  Mention when & what servers you
found using this mode useful with if at all possible.