You’re addressing an important issue, but I do worry about the ways the system will be corrupted. I don’t see a way around it that’s reliable enough.
Meanwhile, we do have other ways of signaling our intent. Candidates count how many “supporters” they have. In my benighted country, they tout how many people have contributed money, and how many of those were small contributors. Polls can ask us who we’re voting for and who we prefer. It’s all very sloppy, but so would a pre-vote sign-up system of the sort you suggest.
Maybe a way to proceed would be to pick a relatively small local election where you think the voters might choose differently if they knew one another’s actual preferences. Do it on a small scale with a relatively low tech approach — explicitly not promising a lot of security — and see if it can make a difference. Maybe?