Accessors: I'd suggest using Class::Struct, although any of the three class-builders would do. The lvalue methods are neat, but I wouldn't depend on them.

Strict Hash Slots: This is trivial if you're using accessor methods -- replace your constructor behavior with something like the below, and any unsupported arguments will result in a "no such method" error:

  my $self = bless {}, $class;
  while ( my($method, $value) = splice @_, 0, 2 ) {
    $self->$method( $value );
  }
  return $self;
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API Delegation: While accessing the internal Net::FTP object is a decent solution, I agree with your instinct to add delegation methods that correspond with the most useful bits of the API, or even to build them on demand using autoload.