You can dynamically switch between normal and reverse serial loopback transceiver modes in Stratix® IV GX devices by using MIF based dynamic reconfiguration.
There is no control pin on the transceiver instance port for switching between normal and reverse serial loopback modes.
To use MIF based dynamic reconfiguration to dynamically switch between these modes you can generate two MIF files; one with reverse serial loopback disabled, and one with it enabled. You can use the standard method documented in the Stratix IV GX handbook to switch between the two MIF files.