For the computer you may be able to use a smartplug to monitor voltage usage but you’ll still need some way of connecting that information back to the Hue. WebCore would be my choice, but since you’ve ruled that out maybe IFTTT…
However, I don’t wish to use WebCore because when I will be moving out from my parents’ house, I want them to have something easy to play with, whether its to troubleshoot something or just add to existing stuff.
This is a very common issue, and people have handled it in different ways since long before webcore existed, usually by adding different devices, such as a pressure mat for a couch or bed or a vibration sensor for a chair.
Some people find that putting a motion sensor under the desk down low catches enough movement from their feet and legs to keep the sensor active.
You can see what other people have done by looking at the quick browse lists in the community-created wiki, looking down near the bottom of that page for the project reports by room section, and then selecting the lists for bedrooms.
I am assuming the issue is that you have the lights set to turn off if no motion right? If that’s the case, also make sure you set the time out long enough that you are more likely to have some motion that triggers in that time window.
I have no familiarity with the Hue sensors, but I have a PEQ Zigbee sensor sitting near my desk and the timeout set to 20 minutes and never have a problem. I would also guess the Iris motion sensors would be sensitive enough to pick up very minute motions, given a reasonable timeout window. In both cases I think the sensor would have to be facing your front and relatively close. That means it can see your arm movements on the mouse and shifting around in your seat.