Did you perhaps see how easy it was to select on the staff menu? It might help us advise other people…
Once the CSA presents their card to the Passenger Operated Machine (POM) and inputs their password, they are presented with the following screen

(The image show numerous options -
PAYG Options Operations, Discount Entitlement, Replace Failed Oyster Card, Manual Charge, Cancel Lost/Stolen Oyster Card, Deposit Cash)
They would then press Discount Entitlements (top middle) and select which Discount they want to put onto the passenger's Oyster Card.
Ticketing & Revenue Book 3 also has guidance regarding Gold Card card entitlement.
(Image shows text as follows:
If a customer has a valid JCP or Bus & Tram discount photocard, a Senior, HM Forces, 16-25, 26-30, Disabled Persons Railcard or an Annual Gold Card, the appropiate discount entitlement can be set on the AFM, MFM or TVM. by selecting the "Discount Entitlements" option.
Holders of an Annual Gold Card are entitled to have the NR Railcard set on their Oyster card, on production of their Gold Card record card or Annual season ticket.
If the customer has a valid Annual Travelcard on their Oyster card, but does not have the record card, you can still set the NR Railcard discount on the Oyster card providing it has the same expiry date as their Annual ticket.)
In all honesty, this is bread and butter stuff for most CSAs at major stations - especially those near mainline stations like King's Cross, Victoria, Waterloo, etc. I would estimate that I could do 15-20 a week sometimes. It takes a minute or two at worst, so whilst I acknowledge the difficulties expressed on these forums getting the proper entitlements added, I am suprised.
The main problem is finding someone with the training to do it - you have a much better chance at Gateway Stations over some single working Supervisor out in the sticks! (I know passengers shouldn't have to go hunting around for the best chance, but that's a discussion for another place, and has been done to death already IIRC.)