Is there any means of retrieving the current pixel coordinates within an image where the mouse cursor is?
(Failing this, are there any stock OS X tools that can easily do this?)
Not that I know of. The closest stock solution I personally use is to take activate the utility to take a screenshot of a screen portion (Command ⌘+Shift ⇧+4). The cursor will turn into a crosshair with the screen's horizontal and vertical pixel coordinates. You then have to apply some math to determine the distance between the end-points you want to measure. Not ideal, but better than nothing.
You can also try using xScope, a nice 3rd party utility which makes life a little bit easier with dedicated tools, but it is zoom agnostic, so make sure you measure at 100% zoom.
Photoshop is the only tool that effectively takes pixel distances regardless of the zoom level, but you have to open the image or at least a screenshot (if you take it from a website or PDF) in Photoshop.