It is possible to access the API for agent management, this API is currently in Preview state, its our Graphql API. We can share the preview docs with you directly if you are interested. It has not been made publicly available as we intend to release a newer API in the future which will supersede this preview API.
This Graphql API doesn’t allow you to pull all assets that could possibly exist in IDR. For instance IDR can identify an asset that does not have an agent installed by observing a Host to IP event through DHCP/VPN event sources. This would be observed in the Host to IP Observations log in log search, if you had a DHCP event source configured for example. You can query these events in log search using the query API https://docs.rapid7.com/insightidr/log-search-api/#operation/getQueryLogs
David