This feature is now available.

In the project edit screen you can now select the level of access for each individual project.

Full project collaboration

Read only

Overview only

Not visible

With Full project collaboration your client will get access to the Kanban board and timeline for the projects. They can add deliverables. You can assign deliverables to them. They can assign to you.

With read only - your client can see the kanban and timeline, but can't make any changes other than add comments on deliverables.

With Overview (this is the previous default functionality) only - your client can see a summary of the project and you can give them deliverables to "Approve". They get a notification, can see the deliverable only once it's ready for approval, and they get a green "OK" button if they are approving of the work done. In this they can upload attachments to an approval deliverable and comment as well.

Not visible will entirely hide the project from the client portal.