So why not create an ASCOM Safety Device to access your API? The Safety Device would be pretty slim and would really just be a way for you to send back True/False to SGP. Just like you're mentioning the "Safe To Image" status could be completely controlled via your API, the Safety Device is just acting as a known device proxy at that point.
If we added the ability to define and directly call an API in SGP it wouldn't make much sense as the APIs could differ wildly. However if you're writing the API interface (the Safety Device) and you're writing the API, then you maintain control of those things and we can expect the Safety Device to act in a specific way. Win/Win!
Best of all we already support Safety Devices so if you were to quickly create one you could have it working by tomorrow 
Jared