Hi, qwertytrewq, a month ago I developed the Idempotent Proxy service using Rust, which can solve the problem you described. I did this to enable canister calls to Google’s reCaptcha v3 to prevent bots.
Here is the canister source code: feat: Integrating Google's reCaptcha v3 into ic_panda_luckypool · ldclabs/ic-panda@add51a3 · GitHub
I also tried applying for a $5k Developer Grant yesterday and noticed that you are applying too. What a coincidence!
I suspect that developers might have developed a similar service even earlier.