This is not first hand experience as I’m not a game dev so don’t have access to the Drops system, but from the documentation it should entirely be possible to do what you intend to do. It’s up to the game dev to decide when to give out Drops:
After you determine that the Twitch user has reached the viewing or broadcasting milestone, you provide the dropped item that is the user’s reward using your own entitlement system.
And that milestone can be anything, it could be time watched, random, or a specific ingame event. One thing you would have to take into consideration is the game client would need a way for the streamer to connect it to their Twitch account, so that when one of these game mechanics is reached you can both check if that player is currently live, and if they are you can then use Drops on that specific channel.