Motherboard panel header current
The sw_pwr is a pushbutton switch input for the power button. Pushbutton grounds that pin when it is pushed, and to sense that it is not pushed it is weakly pulled up to some supply voltage with a resistor that limits the current to few milliamperes. It can't be used to power anything, and it may not a 5V signal either, but whatever the chipset uses as IO voltage.
If you have USB headers with standby 5V available, that could work.