I have a different issue.  I get  revenue via invoices that relates to specific projects.  For example, some have sponsors who provide funding; I have to invoice the donations and keep track of them via the customer/sponsor.  Alas, that means I can't track them directly in the project.  I created a workaround with tags that helps me group them, but I have to add them into the financial calculations manually.  A dropdown for "project" on the invoice creation page would be great.  It might also solve the problems listed above.