David, it doesn't not work now (as long as you ensure DRX is on in the decode to avoid funky highlights) with respect to the camera. We were able to verify the ACES code values we're generating are reasonable and about as correct as you'd expect from any camera given every camera is not perfect so I think you can say Epic works with ACES.
Although the ACES colour space is fixed standard, but the rendering part of the ACES workflow that takes ACES data and makes a viewable image is a moving target as the Academy go through iterations on their rendering and monitor specific output code to achieve the image they want to see. Other than the aforementioned issue of ensuring DRX is on for the creation of ACES data, any rendering issues I've seen are to do with how ACES data rendering in general is developped and not camera specific. Rendering issues are, however, shot specific in that some scenes can appear much more reasonably rendered than others. So ACES rendering may or may not work, but that's not an Epic camera issue. This goes back to what Jim said when he started this - ACES is in development. We don't think that it's suitable to use "in anger" on a real world production today. If you are experimenting with ACES, that's fine, and if you're experimenting with Epic and ACES you really should be in close contact with myself and the Academy.