Right, but 24h is also typically the OLCI window (or close to it) on the OW airlines I've flown on.

So: say I'm flying with hand baggage only, and I have two OW tickets, the first ticket covers aaa-bbb-ccc, the second ticket covers ccc-ddd. They're all long flights, so the departure from ccc is at least a day and a half after the first flight departs from aaa.

I pitch up at aaa and attempt to "through check" to get myself protected, but neither I nor the agent at aaa will be able to check me in for the ccc-ddd sector as it's not open yet.

So, the only way to get anything remotely resembling a "through check" to get the itinerary protected would be to provide an empty cardboard box and insist on it being checked through aaa-bbb-ccc-ddd? I then have a bag tag to wave at the agents at ccc should I misconnect there?

What a massive waste of everyone's time and effort.