Soft specular nodes that respect the environment sphere can be built. I know exactly how to do so.

The specular nodes should ask the renderer to calculate a specular convolution of the environment map. The exact convolution would be chosen on the basis of the algorithm encapsulated in that particular node. Then the node just does a lookup. If there are many different node settings (due to lots of unique materials  in the scene) then you may run into needing many convolutions. However, specular convolution is usually much cheaper than diffuse convolution, because specular does not need to convolve an entire hemisphere into each sample, the way diffuse has to. So these various convolutions are not so expensive to calculate. And, the more broad the convolution, the smaller the map size can be. You only need a detailed map if the convolution is tight.

It can be done - it is a variation on the mipmapping texture filtering technique. Don't say something is not possible in software - it is like claiming that something doesn't exist, solely because you don't know it does exist. Even if you are a software developer, there are others who know different things and can apply information you may not be incorporating into your analysis.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)