Here you are: PsShutdown.
To remotely hibernate another computer:
psshutdown -h \\othercomputer
Note that I didn't test this because I don't have any computers here right now that I'd care to hibernate, but
psshutdown -? lists -h as the hibernate command, and it can remotely shutdown and reboot other computers which I have tested before.
Note that you only need the command installed on the machine you want to issue the command on, not on the one you need to reboot.
Also, just to stave off someone testing this just to see how "fun" this command is, it should go without saying that you won't actually be able to remotely bring the computer back up from hibernation. For that you need to use sneaker-net.