Awesome idea. In fact, I've already got this working. It will be in the upcoming 2.0.4 version.
The button is next to the rotation period properties. Select a body, switch to the Dynamic properties tab and find the text box that says "rotX". (It's not there yet, wait for 2.0.4)
This feature simply sets the rotation period of the selected body equal to its orbital period.
This new feature won't work exactly right if the axis of the body's rotation isn't aligned with its orbital plane, but this is a good starting point.
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Feature ideas are always welcome. Constructive criticism and complaints are also encouraged. As the developer, it's easy to become blind to problems or issues that are obvious to everyone else. How else am I going to make it better if you don't tell me what's wrong?