Hi Andrew,
Thanks for clarifying, my perspective is that you need somewhat a token alike approach. But before that, you still must have a point of reference to that User (i.e. email address or others that you think more related to your requirement) were in you will have to send a link for them to be able to proceed with the voting.
1. Create an entity to store the following:
a. Generated token (maybe use GUID)
b. Email
c. Datetime created
d. Status
2. Send a url (together with the token in the url)
3. Set an input parameter to the webscreen to handle the token based from the url
4. In the preparation, validate the authenticity of the token.
That's what I can think of. You can also wait for other community members for suggestions.
Hope this helps,
Thank you and kins regards,
Chris