It could be a problem.I personally would be apprehensive about mounting them that short a distance from a radiator.
However if no alternative position available how about fitting a radiator shelf over the radiator as they are designed with built in a heat reflective strip facing the top of the rad.
I actually have my rear speakers above the rad.for about 5yrs using this arrangement and have had no probs.with the speakers.The added benefit is you can maybe place some movie memorobilia on the shelf as well!!!
For what it is worth, I have my tv wall mounted about a radiator (with a radiator cover on it) and it poses no problem (for over two years so far). The centre speaker sits on the radiator cover as well.