If you want to see just the Browser Viewer Timeline in your Custom Workspace layout it always opens the Library Sidebar. (That is: the FFEventLibrarySidebarHidden - property is not respected.)
But with the Property List Editor (Xcode) you can edit your .fcpworkspace - file and set the Frame size of the PEMediaEventOrganizerContainerModule to 0 --- the sidebar is gone! ….and your Workspace is the way you want it to be:)
If you don’t have Xcode installed you can use the free PlistEdit Pro.
Extra tip for Laptop users:
You can gain a little screen estate by setting the ‘TB Is Shown’ property to 0. Now the toolbar is hidden.