You may be able to ask support to become a Beta user of the custom object. Once activated, you can see them in the lead detail screen, as a separate tab. But you will ne be able to edit them. Then you will be able to add custom object through te API or with an import.
That might be handy. For now I'm just going to request that our one custom object be made visible on the lead details screen, as you said, but I'll keep that Beta user idea in mind. We may be building some other custom objects in the near future and an easier method for creating and managing them might be a boon to us. Thanks, Grégoire!