What about Jon Lieber as an option? He's signed only though this year and doesn't seem to have a place in the Cubs pen or rotation right now. Though with Wood out they may not want to deal pitching.
He's likely to give about league average innings and would be an upgrade over both Ponson or Rasner.
The Cubs probably wouldn't be asking too much for him but you never know.
If the price was right, I'd like to see them at least explore him as an option.