I'd get a new cable as I can't imagine an easy way to lubricate the entire length of cable. You can always get a speedo cable from one of the guys parting one out as it isn't a very common failure item.
You should be fine on the gauge hood if it's new as the plastic hasn't become brittle in the sun. Just apply even and steady pressure. I've shanged a speedo cable out on a Miata before. Not to difficult.