I personally use afio to back up four HP-UX boxes to one DAT drive. I
don't use cpio often enough to answer your question specifically, but
this is the way I do it with afio:
remsh DAThost dd if=/dev/DAT bs=1024k | afio -i -
The - represents standard input. Using the standard output of a remsh dd is
a good way to go. I do something very similar to back the machines up.
find / -print | afio -o - | remsh DAThost dd of=/dev/DAT bs=1024k
I hope this helps.