Ethernet MDI simulations are analog and consequently IBIS is not a good modeling choice for those buffers. This is why all of the Ethernet controller IBIS models do not include the MDI buffers. The only IC vendor that provides simulation models for the MDI buffers (as far as I know) is Intel. The models are in HSpice format and are using PWL sources. The simulation process follows the Ethernet Physical Layer compliance test procedure (as per IEEE 802.3 standard) that assumes that the Ethernet controller is set in various test modes depending on the operating mode. For more details you need to contact Intel or consult the standard.
Hope this helps,