You can write a simple applescript that emulates the press of the key.
Like:
Code:
tell application "OmniFocus" to activate
tell application "System Events"
tell process "OmniFocus"
keystroke (ASCII character 28) -- left arrow key
end tell
end tell
Then, map it to the sortcut of your choice (ctrl j, ctrl k...) with the launcher of your choice: Launchbar, QuickSilver...
QuickSilver lets you restrict a trigger to the applications you want.
Unless I misunderstood your request, this has nothing to do in OmniFocus.