Re: metal tubing for wvo line
The WVO has done nothing to my copper thus far. Electrolysis might be a concern if I was going to have the system in place for half a century to come, but thus far I'm not worried. To each his own.
I think a few guys on the board here are using PEX. My personal fear is that you would be operating right on the edge of its capabilities in terms of temperature. I would be far more worried about it breaking down from the heat than from steel or copper. The other concern is the emergency fix scenario. The PEX fittings aren't hard to come by, but the crimping tools can be.
I suppose you could use some of the high dollar oil resistant air hose if you wanted. I think that might work just fine, but it's going to be darn near as expensive (maybe more?) than running metallic lines.