A good fit is RawTherapee’s community contest called PlayRaw. It runs as a regular competition and is specifically about taking the same RAW file and developing it in different ways, which sounds exactly like what you want to demonstrate.

Another example mentioned is Jared Polin’s RAW-editing challenge, where people process the same file and he reviews the submissions in a video. That gives you a set of different interpretations plus commentary, although it’s more of a one-person judged challenge than a community-voted contest.

For your specific goal—showing how varied RAW development can be from one file—the RawTherapee PlayRaw competitions are probably the best match, especially since they’re tied to a free RAW workflow community.