Do overland long-haul flights have something similar to SELCAL?
Yes SELCAL is provided over land in remote areas.

I checked the Russian AIP, and shown above is an example.

I'm not sure which FIRs the flight from Dublin to Hong Kong will pass through, mainly because the routing near China is not always consistent. Nevertheless, in remote Northern areas as shown SELCAL is provided.

According to ICAO Annex 10 it will be the operator's responsibility to arrange its use if they want to (or if it is mandated). For example:


  5.2.4.2.1 PANS.— It is the responsibility of the aircraft
  operating agency and the aircraft to ensure that all
  aeronautical stations, with which the aircraft would normally
  communicate during a particular flight, know the SELCAL
  code associated with its radiotelephony call sign.


Regarding the headset, the crew doesn't have to wear it unless mandated by the airline. During long cruise it's not uncommon to remove the headset and turn up the volume of the loud speaker in the cockpit.