I know very little about the CSPPC. I was never rich enough to own one unfortunately

However, it seems to me that you could quite easily tap your 5V directly from the main power supply? It would just involve routing a suitable cable, surely?
This would be the preferred option too, because even if there is a 5V supply available on the CSPPC, you'd be better off not drawing any current from it with external devices. If you do start adding things on to the CSPPC's supply, you risk a situation where the on-board 5V line drops and then you could end up with a more unstable system.
My advice would be to connect any fans directly to the PSU if possible. Then you can use a fan that switches between 12V operation and 5V operation based on temperature as well, if you want.
Apple Hammer