in order for that to work for compression, the attack and release settings of the comp would also have to scale with the tempo. something that I can't imagine would be easy on the CPU (especially when as you say, things are very busy), if it's even possible.
come to think of it, the attack and release settings would have to scale to tempo for EVERY device (including synths), and even sampled instruments would need to be time-stretched in order to maintain an accurate idea of what's going on. I can't imagine this is a remote possibility in the near future.
and I imagine that tweaking EQ while listening to a time-stretched version of something is a sketchy idea, since you'd be EQing a sound that will have artifacts (even if they're not obvious) introduced, only to have them removed again once you've figured it out and start playing back at normal tempo again. at best I'd think that would be good for broad EQ changes, and less useful for precise fine tuning.
that said though, I do think it's a pretty sweet idea! imagine how much easier it would be for newer users to hear how compressors are affecting their sounds. hopefully this happens at some point in the future, but I'd guess it'd be at least a few years off.