I'm currently experimenting with one such idea. Last April, I coated a ferro rod with mineral oil and slipped it into a piece of tubing. It's been with me on several week long outings and has been exposed to salt water spray. So far, there's no damage visible, but I haven't taken the rod completely out of the tubing. In contrast, the BSA HotSpark carried (simultaneously) without any protection is showing some fairly severe pitting - still works, though...

M