I suspect they may get arsey if you try to get away with just a bike box and hand luggage, but if you’ve paid for a bag as well as the bike box then I can’t see any problems with ‘padding’ in the bike box. I’ve just got back from Mallorca with the family and the beach towels, nappies, kids toys, tools, bike shoes and energy gels etc were all in the bike box and weren’t an issue, partly due (I think) to two other bags in the hold and the box and both bags were within the weight limits.