dd doesn't require either if= or of=. It'll read from stdin (or pipe) if you omit if= and it'll write to stdout happily if you omit of=.

From the man page:

    if=FILE - read from FILE instead of stdin.
    of=FILE -  write to FILE instead of stdout.

So you can do:

   cat x.iso | dd oflag=direct bs=4096kB | /dev/sdc