you could loop the pageItems to find the topmost pageItem that is enclosing that coordinate point and return it's fill color (if it's vector).
Haha I think to expand on williamdowling's answer, you could do the following unrecommended workaround all within Illustrator:
1) imageCapture the entire document (extra work if you have multiple artboards, but let's assume it's just one for the moment) and place the captured image into the artboard on top of everything else.
2) Use a clipping path to clip off a square at the xy position.
3) Rasterize the clipped item.
4) Use image trace to convert this clipped square into a vector colored square.
5) This vector square will now be filled with your searched color! This color may vary and is not to be relied on. It will have been ran through multiple conversions, even more so if the document color space is CMYK, and may have little in resemblance to what would be the result using the UI color picker manually.
you commented on this exactly a year to the day from my previous comment.
not important. just vaguely interesting.
au contraire, I do believe it was on the same exact day!
oh. i'm an idiot. i SWEAR it said 2016 for your post.. i wonder why this got bumped to the top of the forum then... the fact that it was at the top of my list bolstered my belief. though i now realize that it hasn't even been a full year since i posted that.
I'll just crawl into a hole where my stupidity won't be inflicted on others.
DO not despair, I think these get bumped to the top when someone find them helpful or likes things in them.
Yeah don't worry, we won't judge you