Have the streamer own the extension and/or the streamer can create an organisation on Twitch and add you as a developer to that extension.
The cut will still happen but the streamer owns the extension they get the cut.
You just might have some fun during testing/running/updating the extension since you are not the owner.
However if all your extension does is just cheer, then you may fall fowl of the “reselling a Twitch product” rule/guideline. since what you describe is basically identical to cheering in chat. There is no added value for the user by doing it via an extension