I don't know if eDP allows a lower speed option, but DisplayPort uses fixed data rates, and then only uses the data slots it needs to send pixels. Even 640x800 is sent using a high speed link.
The rates are fixed at 1.62Gb/s, 2.7Gb/s and 5.4Gb/s - all outside of the rates achievable with standard SERDES pins, even if they were electrically compatible.
Unlike HDMI, because it uses Clock Data Recovery I expect you will not be able to establish a valid link if you try to stray more than a few % from these speeds - the receiver's PLL will not be able to lock successfully.