iftomkins - We are already doing this in our community where we store the required details of the user in the custom field rather than the cookie.
Regarding the custom field, at the moment this can be added via Li Professional services only, and that is at no additional cost, we have got many custom fields for our communities. They will set it up on your staging instance for review and then once you confirm, will be pushed live in your maintenance window.
Your custom field will be visible in the user profile data in the API response and will also be available in the metrics reports.
This is how you can access the Custom fields:
https://community.xyz.com/restapi/vc/users/id/[ID]/profiles/name/CUSTOM_FIELD
This is how you can set the value in the custom fields:
https://community.xyz.com/restapi/vc/users/id/[ID]/profiles/name/CUSTOM_FIELD/set?value=VALUE_TO_BE_SET
This works really well and would suit your scenario of showing the form only once to the user.
