We're not talking about overtime payments, just pay accrued during normal, contracted hours.
I would say every CC would want sector pay/hourly flight pay to be paid during leave periods, however the picture for allowances would require more careful thought. In most airlines, allowances are partly taxed, so a calculation must be made to see what delivers the best net financial position bearing in mind the potential tax issues.
Be under no illusion, the days of tax free allowances paid at a rate any higher than the actual expense incurred are rapidly coming to an end, BA judgement or no BA judgment.
As for pay to fly stuff, of course, that is a far bigger issue but separate issue, however pilots in airlines such as easyJet where as I understand it a significant proportion of remuneration is activity based, will have perhaps the most to gain.
This issue is not just about BA and could have a positive outcome for most if not all UK pilots.
Fingers crossed the judgment goes BALPA's way.