Actually you can do that with just the kernel and grub. Put a kernel image and a statically linked busybox on a usb drive, install grub, and add a line as follows:
kernel=/<kernel image> root=/dev/sdb1 init="/busybox echo hellow world"
Booting the above will accomplish what you want. Although I don't know if init=
accepts the argument that I've written here, and because after doing this the kernel will panic because our "init" would exit, it is better to make a small script that would call busybox as echo first and then sleep indefinitely.
EDIT: Also the above assumes grub 0.99 and that the usb drive is the second disk on the system (the local hard disk being the first). Either use values that correspond to your setup or just use UUIDs