I don't have a spare disk to try this out on, but you can use the yes
command to continuously push your string into the pipe:
yes "Hidden" | dd of=/dev/sdb
I assume once dd has written the whole disk, then it will close the pipe and this command will finish.
The above will newline-delimit the "Hidden" string. If you want it space-delimited, as in the question you can do:
yes "Hidden" | paste -d' ' -s - | dd of=/dev/sdb
Or if you want it null-delimited:
yes "Hidden" | tr '\n' '\0' | dd of=/dev/sdb