Reverse Serial Loopback Datapath

Hi,

I'd like to know if there is way for introducing a module in a Reverse Serial Loopback Datapath, post CDR, between the Receriver PMA and Transmitter PMA.

In the attached drawing is easy to see what I want. In a GbE link I would like to loopback the serial communication and introduce errors according with pattern.

I'm using serial_altgxb_alt4gxb core, if someone could point me out the signal where I could hook my device, I'd be really grateful.

Cheers,