The RGMII_OUT registers are implemented using Alt DDIO_OUT atoms which cannot be observed using SignalTap as a routing path to the core is not possible. Hence it is not possible to signaltap these nodes.
In the SignalTap II Logic Analyzer, the RGMII_OUT signals are only visible by selecting SignalTap II: pre-synthesis as filter, however after compilation, from the Compilation Report > Analysis & Synthesis > In-System Debugging, you will find that the status of the RGMII_OUT signals are illegal and these signals are connected to GND.
If you change SignalTap IIs filter to SignalTap II: post-fit, you are unable to find the RGMII_OUT signals.