I wrote an Applescript to handle cases like this, which I've attached to this post.
See this thread for details of how to install and use Applescript with Omni applications:
http://forums.omnigroup.com/showthread.php?t=7453
To use my script, you just select the repeating action you want to temporarily reschedule. It makes a duplicate of the action, which you reschedule as desired. The duplicate will not repeat, and the original is altered to fire when it would next be scheduled on the original schedule. This way you don't have to remember to reset the start date before completing the current action.