I've checked all the documentation but there is no mention of voltage on the fan header.
Could someone please confirm, I don't want to destroy a 5v fan finding out...
It is a 5V pin header for the FAN
Follow-up question: what is its current rating, and can I use it for something other than a fan? Specifically, I want to drive some (a small number of) smart RGB LEDS (yes, I know I can't drive Neopixels directly from Linux, I will also be using an Arduino Pro on the UART port; got it covered). However, these use a lot of current. If the fan header is basically just connected to the 5Vin, but it would be nice to know how many amps the internal traces are rated for. On the other hand, if the fan header is a PWM or controlled by the thermal subsystem, this would not work and I'll need to distribute power to the LEDs via some other mechanism.