UPDATE: Note that we access the API’s server side as the connect app user, not trying to run as the guest user itself.
Hi @sax , we are similarly waiting for Atlassian to add full support for guest accounts to the API. (Guest users not visible in /api/search and are not External Collaborators)
We did notice recently that guest users are now returned from the /rest/api/search API (https://developer.atlassian.com/cloud/confluence/rest/v1/api-group-search/#api-wiki-rest-api-search-user-get). Enable the sitePermissionTypeFilter and it will return guest info. Note there is still a bug that the externalCollaborator flag returns false for guests).
If you have the Guest user account ID, try something like this:
https://{{SITENAME}}.atlassian.net/wiki/rest/api/search?sitePermissionTypeFilter=externalCollaborator&cql=user%3D%22{{ACCOUNT_ID}}%22
Chris