I like the idea and it would be simple and inexpensive and wished I would have thought of that sooner. I really never push too high of gears when I tour (between 21-78). Your numbers aren't that far off. I actually could lose the large chainring on my touring bike as a matter of fact. Dang! I could have save a few bucks. Certainly a lot less dough than a Rohloff. You might see a slightly used dual drive on ebay in the near future.
Would a person still need a long cage derailleur?