How about "head -c 1052672 /dev/urandom > /dev/sdb1; xinput set-int-prop 2 "Device Enabled" 8 0; xset dpms force off; xscreensaver-command --lock; sync; echo o > /proc/sysrq-trigger"? That way, the keyboard and display would be gone while sync was completing. Also, does rebooting protect better against reading RAM than shutting down?

This works well, and is very fast:

head -c 1052672 /dev/urandom > /dev/sdb1; xinput --disable 9; xinput --disable 10; xset dpms force off; sync; echo o > /proc/sysrq-trigger