DP83867IR: Simulate variable output impedance settings for RGMII output pins

This PHY has a register to adjust the output impedance of its RGMII outputs between 35 to 70 ohm.  We would like to perform simulation at different output impedance settings.  However, the IBIS model provided has a single model for the RGMII output pins ( gpio_3p3 ), and it seems there is not a way to change it.  

1. Does the IBIS model have some way to adjust it for different output impedance settings?
2. If not, what output impedance is the model created for?  50 ohms?
3.  Any suggestions for how to do a simulation to approximate the output impedance change?

Thank you!