Ambidextrous backup downtube shifter for touring?
We need to see what is under the cable stops. My guess is that is is probably a standard squared off shifter boss, but you never know. If it looks like this



Then any friction downtube shifter should work as a backup. You want to get
one that is not indexed if at all possible since that way you can use it for 
either front or back derailleurs. Bar end shifters use the same mount, so if you have some of those handy you can see if they fit. The cheapest solution is probably to buy a cheap pair of Sunrace Clamp-on stem friction shifters.

While the levers are different, they can be interchanged. You should probably test this in case there are clearance problems and you need to trim the lever 
a bit. 

As to whether this is worth the cost and weight, I'd say it depends on the remoteness of the tour. If you're within a day's ride of a bike shop for the
entire tour, then it's probably not worth it. A credit card makes a handy backup
for a lot of parts.