Not sure about any such products. Mainly because most home use routers are very limited with respect to such controls.
DD-WRT and Tomato are firmware programs that can be installed on many make and model routers. Perhaps one of those programs may provide the granular control needed. Main risk is that the firmware installation process can "brick" the router to the point of uselessness. Overall, the installation process must be done with planning and care. Perhaps another forum member can address any connect/dis-connect options within DD-WRT or Tomato.
The last "solution" that I was addressing involved using PowerShell and remote access to stop and start a process on the targeted computer.
However going a step further, I can envision a simple menu screen via Visual Basic, Powershell, or some other tools that would list the targeted devices each with a green (on) and red (off) button. Would work by remotely starting or stopping some process on the target computer. May tinker with that if I get the time over the next few days. Would be intranet only I think. Just seems cumbersome and all in all messy.... But if it can be made to work on one computer then the buttons would simple by cloned and the code edited for individual devices - maybe using the MAC.
Good project for programming class....
That all said, I still believe that Parental Controls will work. Block any given computer 24/7. When contacted you would use the day/time settings to un-block the desired time span. My Linksys has 24 one hour blocks per day.
Parental Controls would always be on 24/7 with all targeted devices blocked except for the time block you have permitted.
Once the end user is out of that time block then you would need to be contacted to extend or shift the time block. Which would be done via the router.
If the router "parental control" option is just not it - no harm, no foul. And no offense on my end.
Will keep thinking about it as circumstances warrant. Keep brainstorming.....