The A300 had access to the forward belly hold via the under floor avionics bay below the cockpit. There was a hatch behind the captains seat to get below and there were two doors from there into the hold. Most of the time you wouldn't be able to get anywhere as the hold would be full of containers. I remember frightening the loaders by opening one of the doors one day whilst doing a pre-flight check.
I think the DC-10 had the same arrangement,
I don't know if newer types have such access.