Is it okay to briefly exceed a resistor's power rating in a high-reliability product?
The data sheet should mention the peak power or current capabilities of the resistor. It is usually much higher than the mean power rating, but if it is not specified anywhere, I would rather not rely on it.

Note that not all resistors of the same size and power rating are created equal. There are some that are designed (and specified!) for peak power ratings well beyond the mean power rating, and there are some that are not.

I have seen resistors fail when their mean power rating was exceeded just briefly, so it's indeed necessary to check what manufacturer and type you're using and if it has the right specs.