You cannot be called in on promise of future payment and I suspect neither can you be self employed to do the same work. If you’re furloughed you do no work for that organisation, full stop. The community/cultural/charity sector have been ‘volunteering’ in each others organisations. In theory aviation or any other sector could do this but I suspect the complexities of it are too great to be worthwhile, making sure a museum is in order is a bit different to flying someone else’s Airbus