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There is a Focus menu item in the View menu, and it is bound to control-command-F. The obstacle encountered if you want to bind it to a different keyboard shortcut is that the name of the menu item constantly changes to reflect the project(s) you have selected in the sidebar, rendering the System Preferences approach helpless.

I am unconvinced that anyone looks at the list of projects to be focused upon in the menu, especially given that after only a few project names are selected, OmniFocus (or the menu code) snips out the middle of the text. Changing it so that the menu item was a constant "Focus on Selected Projects" (as it reads when nothing is selected in the sidebar) would not appear to do any substantial harm, and would make it possible to rebind the shortcut.