Super resolutions will work in any condition. As for the desktop, if you're lucky, 640x480i might work (12.5 MHz). If you're not lucky then try width double width (1280x480i, 25 MHz), this will work for sure.
There's one limit that applies to the DP driver's implementation, and another one that applies to the dongle chip. I may be wrong but this is what I gathered from my tests. But it's been a year since I touched it.