Is there a (programmatic) equivalent to Linux's "Alt-SysRQ Sync Unmount reBoot" reboot sequence on Windows?
Regular shutdown causes a lot of data to be written to drive, yanking the power cannot be done remotely.
Yes, there are undocumented NtShutdownSystem(), NtSetSystemPowerState() functions exported by kernel and they can be called from userspace: http://www.codeproject.com/Articles/34194/Performing-emergency-shutdowns
Beware: this is unclean shutdown as you requested, so you may lose your recent changes, corrupt registry etc. Use only if you need to reboot remote server, and something prevents normal reboot from happening.