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You can set this up with the Keyboard panel in System Preferences. Select the Keyboard Shortcuts section, select Application Shortcuts on the left, click the + under the right hand column, select OmniGraffle in the list of applications, put "Print Canvas" in the Menu Title field, and type your desired shortcut into the Keyboard Shortcut field. Click OK and try it out.

As for wasting paper, you have to confirm the print dialog before it actually does that, don't you?