At the moment, without that functionality, you could use Keyboard Maestro for that.
I believe though, this would be a nice addition (and not hard to implement, as it is currently present on the right-click menu) and suggest you to submit your idea as a feature request, by clicking Help>Contact Omni in Omnifocus — currently, the only official way to request support/submit a feature request is via email :D
Also, it would be nice when the icons for Parallel or Serial that are to the left of the Project or SubGroup would function as toggles. I find myself so often clicking on them with the hope that I can immediately toggle a Serial to Parallel or vica-versa.
I guess the preferred email is actually omnifocus@omnigroup.com, as per Help>Contact Omni — but I guess we’re talking about the same thing (I was just showing how to fish ;-)
There’s an existing feature request to add keyboard shortcut support for toggling project types as you describe — I’ve let the rest of the team know you’d like to see this, too. Thanks for passing that along!
I’m also looking for a shortcut to toggle parallel and sequential project type. Even if it’s as simple as adding this to the menu bar so I can set my own keyboard shortcut that would be great.