To assess loss of productivity you would have to rely heavily on the accepted programme and the clause 62 programme for the CE.
Essentially its no different from assessing the impact of any other CE. You are assessing the effect of the compensation event upon defined cost and the effect on planned completion - the actual defined cost up to the dividing date and a forecast thereafter.
Your loss of productivity would I assume largely fall into the forecast of the work not yet done. So for example (in very simplistic terms) if the remaining work, prior to social distancing measures, was due to take 4 operatives 4 weeks to complete and now due to social distancing measures will take 2 operatives 12 weeks, the effect upon defined cost would be the cost of 8 operative weeks plus the cost of planned completion extending by 8 weeks. The same basic principle applies to all cost components.
The clause 62 programme accompanying the assessment of the CE should sufficiently show the impact on the activities so that you can understand the underlying causes of why the work is now less productive. I agree with you this type of assessment is often very subjective so to avoid quotation “table tennis” it’s best if the Contractor and PM sit down and speak to each other to about what the best solutions are for the project.