Ok, I just had to go ahead and try this myself
I created a bunch of nested symbols with overrides, and used their IDs to build this thing:
var layer = context.selection
var path = "21A3FCD7-83C0-4601-8527-67AC81E99828.2AB56A50-4051-418D-8301-DC0A3A0ADB3C.6F98D4B3-0947-4C35-85CF-0214A7DC6F4A"
and it did log the expected value for that key.
allValues().objectAtIndex(0) bit is needed because the overrides are contained inside a dictionary which is inside another dictionary. This will be cleaned up in Sketch 44, IIRC, so the code will be a bit cleaner (you'll just need to use