If I understand you correctly, I've been wondering something along the same lines. I just started using DSP presets in connection with the DSP field recently, and I've been very happy with the results. However, I was somewhat disappointed to find that the EQ settings did not return to the default when a file came up that did not have a value in the DSP field. Of course, the obvious answer was the set the DSP value to "Default" for every file for which I had not entered a value, and to automatically add that value to each file during the import process... but it does feel like an unnecessary extra step.
It had also occurred to me that it would be brilliant if we could add an expression value in the DSP setting. For instance, I have DSP presets named things like "Audio.Music.Rock.Prog" or "Audio.Music.Pop". It would be incredible if I could set the DSP field to something like "[Media Type].[Media Sub Type].[Media Type].[Media Sub Type].FirstNotEmpty(Replace[Genre],\,.), Default)". (Forgive any syntax errors there, I didn't test it... You get the idea.) Is something like that possible? Obviously I could set a static value at import, but I'd like to be able to just enter the expression in the DSP field; that was, if/when I set a new Genre value for the file, I wouldn't have to remember to change the DSP value as well.
And as long as I'm throwing around stupid questions (or wish-list items), is there any way to find the list of available DSP presets in the expression language? It would be really cool if I could check to see if the result of the expression is an existing preset and fall back to a default when it is not.