It depends. Usually fans pull air from the front of the case, and blows it out the back. If you put the back of the computer towards the radiator it should be fine. Just make sure you can feel the air being pushed out the back of the computer.
My problem with that is that I'll have to
drag the PSU wire around the tower to the cable management hole on my FITTED desk. There is a power outlet which I got an extender cable to reach behind the bed. Would it be ok to put the PSU cable past the radiator to the extension cable?
Yes. The power cables are usually insulated enough that they don't get very hot. You could maybe use some tape, conduit, or cable holders to direct the cable so it gets the least amount of heat from the radiator. Also if you can cross under the radiator(on the floor) you'll probably not get any affect on the cable as heat rises.