This idea has three inspirations:
1. Axle housings. They have oil and grease on the inside, one vent to relieve pressure so the oil seals don't suffer it, and I've never seen one rusty inside.
2. Rocker panels without rust holes. They have rubber plugs and appear to be semi-airtight. I've seen some very faint surface rust inside them, with the outside looking far, far worse.
3. Another forum member who built a flatbed frame/cage for their truck out of square tubing, and made it airtight enough to work as a compressed air tank. It sounded like the "making it airtight" part was no big deal compared to making the frame in the first place.
Suppose I install a humidity and maybe a fine-scaled pressure gauge, so I can monitor the frame over time and notice when it starts leaking? Unlike spraying various sealers inside, touching up some spot welds that let go won't be too much trouble later on.
Thanks for the feedback, guys. Any thoughts on my frame-outside plans?