This is a super interesting idea!

For potential donors who are risk-averse or financially insecure and unsure about their first few donations, I think this will significantly boost willingness to donate. I'd also expect redemption rates to be pretty low. All this strikes me as feasible to test on a limited scale. Some implementation concerns:

(Asking anyone who might know) How much might this complicate funding allocation/accounting/planning? I'm not 100% privy to the inner workings at Givewell or OpenPhil, maybe this will increase funding uncertainty and planning. I'd assume that any significant implementation would grow to comprise most donations, since it's better to have the option than not. edit: I'm assuming the emergency fund would communicate/integrate closely with funds themselves, since it would seem smoother and more secure that way instead of operating 100% independent

What happens in financial crises with abnormally high withdrawal rates? Short-term crashes are practically inevitable, and I'm imagining the headache that comes with a "bank run".

I'm pretty sure some nonprofit somewhere has thought of this, and come up with useful answers or other creative implementations. Perhaps a solution is to simply provide a warranty? (i.e. no questions asked return policy with 2/3 years) This way, funds can maintain an element of financial certainty, and they can give the money back at their discretion anyway.

I'm curious to discuss this more and lend my limited fundraising/entrepreneurship/UX perspectives. I think the premise itself is promising, and plays on the EA community's strengths of mutual helpfulness and goodwill.