On monitoring, one reason for optimism is that it can be done in an automated way, especially as AI capabilities increase. But I agree that it might be possible to hide misuse. Looking at the user's queries and the model's predictions won't always give enough information.
On a common open source system for structured access: yes, that's something I've been thinking about recently, and I think it would be beneficial. OpenMined is doing relevant work in this area but, from what I can see, it's still generally too neglected.
This is a linkpost for: https://www.governance.ai/post/sharing-powerful-ai-models
On the GovAI blog, Toby Shevlane (FHI) argues in favour of labs granting "structured access" to AI models.