Measuring Power Consumption of a Mixed-Voltage Device
It would be easy enough to make an "extension" cable for the 24-pin ATX power cable. Into this special test cable, you could wire "high-side current sensors" that would measure the current for each supply voltage. With this test/monitor cable you could easily connect it to any power supply and any motherboard.

These are inexpensive enough that you could properly meter each voltage. And they are easy enough to interface to with laboratory instrumentation, or even a hobby microcontroller like Arduino, et.al.

Here are two examples of high-side current sensors. Already assembled on "break-out boards" and ready to wire into your test circuit. They are both &lt; US$10 and there are several other examples of similar products.

https://www.pololu.com/product/1186

https://www.sparkfun.com/products/12040