From AwkwardTV

[edit] For the Repair Volume

Mount the repair partition.

mkdir /mnt/repair/
mount -t hfs /dev/disk0s2 /mnt/repair/

Edit /mnt/repair/com.apple.Boot.plist: under key Kernel Flags, add

-v

before rp=:///OS.dmg

[edit] For the normal OSBoot Volume

Edit /System/Library/CoreServices/com.apple.Boot.plist: under key: Kernel Flags, add

-v

Thanks to phoem for this information.