From our knowledge of the AgentVI integration: AgentVI has developed a plugin that pull information directly from an AgentVI video analytics server, it does not pull the information from the XProtect server. This means that you cannot develop an integration where you get the information for the overlays from XProtect.
It might be a good idea for you to contact AgentVI and ask if their servers have a public API.
PS. It would be a good idea to contact AgentVI also to confirm the mode of operation for their integration.