Rust inside + good idea?

Spray some paint inside there, enough so you can slosh it about and cover everything. Or get some ptfe based spray and spray a load in from time to time

Yeah, I'd say paint sounds like a good idea. Just last week my buddy went to smith stall a quarter and his forks snapped at BOTH welds! Turns out the steerer tube near the welds were rusted real bad.

A good rust inhibiting paint, like Rustoleum, would work but it would be hard to evenly coat the inside. If you wanted to do it really cheap, go with WD-40. It could be a little messy if you apply to much, and you will most likely have to coat it every season.
The other option is frame rust inhibitor. It's made specifically for your problem and cost about $13 a can. It's popular with the bike touring crowd, mainly because they are the last bike subculture other than BMX that loves steel frames... and they ride in any weather.

lp3 rust inhibitor in a spray can will do the trick. I use it on engine parts that are sitting around so they don't rust. It's kind of expensive though but it sticks and works great. Make sure you get it good and dry first before you apply it.

It's called LPS3. Not LS3. My bad.