Is it possible to shutdown the network computers from the router running Tomato ver 1.28 on a Asus wl-520gu?
No. And yes.
No: There is Wake-On-Lan standard, but no corresponding Sleep-On-Lan.
Good thing, too. Imaging the havoc a virus could create.
Yes: With cooperation. Your network computers could periodically sniff a remote or shared file folder for the existance of a flag file, and then shutdown when it is detected. Your router could create this file whenever it wanted the big computer to shut down.
Another "yes" … with scripting and an SSH server running on the computers. (Yes, Cygwin's ssh running as a service will work.)