From a legal point of view, not just a Twitch Extension Guideline one, your extension idea has nothing whatsoever to do with ‘donations’.
Donations have a specific legal meaning, so unless both you the developer and the broadcaster are registered charities then none of that is a donation, but a ‘tip’, and even as a Tip this still would be very unlikely to be allowed for the reasons Barry has mentioned.
Also, from a usability point of view it would be unlikely any streamer would use such an extension as there is a 80/20 cut for extension transactions, so 20% of the bits would go to the developer, which for an extension focused on users tipping the streamer it would be much better value for money for them to just cheer the bits so that the streamer gets 100% of them, and then the streamer can just use on-screen graphics for progress bars, or use chat messages to say the current goal status.