Right way to couple DGND to Chassis ground
Typically mains ground is your best bet, assuming that the connection to ground is good (in the building) and the wiring in the building is good, it's as good as you can get. A ferrite on ground will actually give you more problems, because any impedance that restricts current on the mains ground will create a voltage on the ground plane.
Make sure the connection between ground and the PCB ground is low impedance (that means low resistance) to prevent common mode voltages.

The System ground and earth are usually capacitively coupled.

No, in all commercial products with no isolation, the ground is ground and should be for safety reasons.

I have seen AGND and DGND coupled with ferrite beads - primarily to
reduce HF coupling yet keeping the DC bias the same. So capacitively
coupling the DGND to Earth is to make sure DC power does not flow into
the system but the HF components flow out of the system?

A properly designed board does not need separation between AGND and DGND. It's about managing currents on the PCB. Currents create voltages through the ground plane. Currents usually arise from bad placement of connectors that are carrying currents or shield currents (shield currents should more often then not directed to chassis ground).
A properly designed board does not have switching return currents running through the analog section (the currents from the GND pin of an IC will return to ground, usually through the shortest distance, and will create voltages in conjunction with the resistance of the ground plane. So place components with large or switching return currents away from sensitive analog components.
Another source of currents is EMI capacitivly coupling with the PCB, and returning to ground in which proper shielding would take care of that problem.