Here's a random idea for you gurus:
Think the Rio Euro Nymph line could be used as a running line behind a Godshall head on my Anglers Roost 11 2/3? It's a really low diameter (.026 at the belly, .023 at the tip) double taper.

That way you could go back and forth between nymphing and casting super easy...

Sure you can. In my DH trial period, to limit the costs I also thought on something similar.

My first running line was a WF5 with a diameter of ~ 1 mm on which I really liked the coating. It worked pretty well. The advantage I've seen over the thin running lines I've tried is that it gives you a little more control, and I can overhead cast better;

But with this type of running line you will loose at distance;

This line was designed to stick in guides, not for better runnability (as with proper running lines)