After a session with Zoho Support, it appears, that it's not possible to enable Client Approval of timesheets that are created within Projects.

I have to assume other Projects users have the same need, i.e. Client Approval of timesheets in order to invoice.  If so any smart ideas on how you're achieving that?

My hope was that once timesheets are approved in Projects, they would simply flow to the Client Portal where the client can approve/reject, and then we'd invoice.

But oh that it was so simple!