I too have done a few and advised others.
Here is an extract of an email i received 3 weeks ago re the DVLA updating V5's to include missing information:
After waiting 6 weeks for the DVLA to review my V5 and update it with information from the CoC, they have returned it unchanged. When I initially called them I was told there would be no issue updating the V5 with information from the CoC.
In the letter I received from the Casework Unit, it states:
The CO2 emissions rating for a vehicle is provided at the time of first registration. If the CO2 emission rating is not presented at first registration, it cannot be included at a later date, even if a CoC or a voluntary IVA cert is present.
YOU will note the email refers to CO2 but the body of the email then refers to all the other readings includin NOX.
It seems then that there is no set process with apparently ad hoc decisions being made.
I asked why he wanted his V5 updated when he lived in the ULEZ zone and his response was to ensure he was compliant for other cities should he visit them.
The tale ends with a rejection by TFL and a couple of hours late a letter of compliance.
Wierd eh!
I've just had a similar experience - sent off my V5C with the CoC and covering letter as per James' suggestion, but received the unaltered V5C back from them along with a letter (in separate envelope - glad to hear our taxes are being spent efficiently!) from the Vehicle Casework Unit which said the following:
"I have taken further advice and have been advised that as your vehicle was manufactured before 01/01/2001 we are not able to update any details on our records.
Even though you have provided evidence of the emission details, updating the DVLA record will not make the vehicle compliant with the Ultra Low Emission Zone (ULEZ) or Clean Air Zone (CAZ)."
So, even though we all know that the second sentence is incorrect (where the CoC shows figures which are compliant), the first sentence indicates that they are unable or unwilling to update the V5C, unless you get lucky and someone who picks up your request doesn't "take further advice" and just goes ahead with the request.