Just a wild guess (I don't have my N900 here atm):
Have a look at:IF there are any devices listed, one of them MIGHT be the IR LED.
If so you should find a file called brightness which accepts values of 0-255.
Assuming the LED supports different brightness levels (most of them only switch on and off) you can check the result with any digital camera (photo, video, webcam) pointing at the N900's IR emitter since their sensors partially cover the IR spectrum.