Yeah, if weight was the only consideration, they would just be hollow. I think titanium valves would be a better idea if price isn't an issue, less weight & more durable.

A sodium filled valve is a good choice, though. The reactivity issue isn't really something that concerns me- if a valve breaks I'm going to be pulling the engine apart anyway, I wouldn't mind a little extra damage if the little bit of sodium in there made them last 3 times longer than a regular one.