As far as I can tell, none of the resolutions over 640x480 have ever (reliably) worked. Otherwise I'd have been happy to use them. I have fiddled with the DC's video settings myself to create a VGA 640x360 widescreen mode once, but the reduced resolution resulted in a frame rate that was apparently too high for the TA to keep up with.
But don't despair! Intrepid Izzy already has an option for letterboxed widescreen.
I do own a DC SD card reader, but I haven't (yet) coded SD card support into the custom DC OS that Senile Team's games run on. I might do that at some point, but for the time being, there are plenty of other things to focus on.