I have thought a little bit about soft-proofing but not much. The basic idea is pretty simple – it’s a matter of changing the output color profile used when rendering images for display or output (but there are some gotchas that have to accounted for or the results will be incorrect). Embedding a profile is also straightforward.
It’s not on my near-term list, but it is something that is on the list for a future release. There are some very important (and large) features that are taking priority.