Before I give you the answer let me remind everyone that you can do so using Project Professional 2010.
So let’s assume I have created three Project CF, we will leverage the new Project Detail Pages (PDP) capability to create a custom page and will add a web part that will enable editing these fields:
The three newly created CF (Server Settings –> Enterprise Data –> Enterprise Custom Fields and Lookup Tables)
Let’s now create a new PDP, go to: Server Settings –> Workflow and Project Detail Pages –> Project Detail Pages
Click on Documents Tab, and click on New Document, enter a Name and Select a Layout
You should now be in edit mode for this newly created SharePoint page (if not click on the upper right corner under Site Action -> Edit Page) . Click on add web part and select Project Web Access –> Project Fields and add it to the desired zone on you page.
Now let’s edit the web part to specify which project fields will be displayed and hence editable. Edit web part and select Modify to select the custom fields you want to be able to edit. Apply changes to the web part and Stop Editing the page
Now let’s add that page to the Enterprise Project Type you want to have that editing capability for. Go to: Server Settings –> Workflow and Project Detail Pages –> Enterprise Project Type (EPT) and select the EPT, probable the Basic Project Plan in your case. Scroll down to New Project Page/Project Detail Pages and add the newly created page from the steps above. Save changes.
Go back to the Project Center and select the project plan you want to edit, the newly created PDP should now appear on the left hand navigation, click on Edit and voila!
This procedure also demonstrates the power and flexibility of the new Project Detailed Page infrastructure combined with Enterprise Project Type!