RGMII isn't supported in the transceiver IP for C10GX. If a user wants to implement their own version in the LVDS IO running at a really low rate (around 125Mbps) is their any reason they can't? Were the timing failures in the IP blatant? Or were they silent failures?
The RGMII was not supported is because there is no way to meet timing. You are supposed to see the timing violation from the timequest if you have constraint design properly.
For the LVDS IP implementation, you can refer to the TSE IP, it did support the LVDS I/O interface when you select the 10/100/1000Mb Ethernet MAC with PCS Core variant.