[Gabriel Radic]

Here is the only workaround I can thing of:

Use a scriptable launcher to insert the line-break character as text. I recommend Butler, but there are others.

You can assign a keyboard shortcut, and you're set. The good thing is that this works in many other cocoa apps. The bad thing is that it enters a paragraph break in Carbon apps. But the Carbon apps already have a shortcut for this. Just like all Cocoa apps besides OmniGraffle :-p