Are you sure about that?

While you are correct in what a TA does, it does not represent what the user experiences when you move to linear pull brakes with a TA. Remembering that context matters, my comments pertained to the OP's proposed configuration of using a 5.6:1 ratio road brake lever to control a linear-pull / V-brake designed to work with a brake lever using a 2:1 ratio compared to how that same configuration works when you replace the V-brake noodle with a travel agent (TA) .

Perhaps I'm missing something, but the way I've come to understand it, a TA significantly increases the amount of brake arm travel (which you correctly note) for the same amount of brake lever pull, and also increases the braking force once the brake shoes contact the rim when compared to the amount of hand force it would take without the TA.

This increase in power for the same given braking force is what gives tandems equipped with TA's what is often characterized as a 'mushy' feel compared to how road brake levers matched to a proper caliper feel. This same initial mushy feeling vs. the higher resistance experienced on a road brake lever /road brake set-up with lower mechanical advantage is why road/road devotees knock the road/v-brake + TA set-up for having poor modulation / tactile feel. As you'd expect, while this is initially true, most folks who have v-brake equipped tandems quickly adapt to the 'feel' and enjoy all the modulation they'd like. However, in either case, it takes less hand effort to achieve the same net braking force (at a somewhat lower amount cable tension) that it would using either the road/v-brake without the TA or even the road/road caliper brakes.

Again, you are also correct in that placing a TA at the lever-end of the tandem would halve the amount of cable tension over nearly the entire length of the cable run, which is why Ibis (with their LOVE unit) and even KHS mounted their cable pull adapters at the handlebar end of their road tandems... and what a gawd awful looking mess that is. TAs have also found a place with some of the tandem teams using a rear disc to mitigate the need to maintain tight tolerances between the rotor and pads give the way TAs modify road brake lever pull.

Frankly, you won't get a compelling argument out of me in favor of V-brakes with TAs for road tandems. I'm not a fan and my first hand experiences are limited to test rides and trying to get v-brake tandems from squealing like Banshees. Instead, and because we're a lightweight team, normal road brake calipers operated by road brake levers is all that we've ever needed, augmented with a rear disc in lieu of the rear caliper when we head to mountains where we expect to encounter technical descents. If we were a heavier team or did loaded touring we'd likely use cantilevers coupled with a supplemental disc or drum brake operated by the stoker for truly demanding terrain.

I would note it's refreshing to see that Dan T's article on brakes hosted on his shop's (R&E) website has apparently been recently updated relative to disc brakes whereby it is far more objective than previous words that have appeared in this same article. I tend to agree with most of what he's saying about discs at this point... assuming we're talking about a rear disc. I'm still not sold on dual discs, but that's a personal bias that has more to do with how a road bike 'should look', added complexity, weight, cost and serviceability when touring, etc...

As Retro Grouch has alluded to, the very tight tolerances the OP will need to get a V-brake to 'have sufficient travel' for acceptable use takes a lot of attention to achieve and maintain. Moreover, once so-adjusted, these tight tolerances makes wheel removal nearly impossible UNLESS you are using them with Campy Ergo levers where the brake quick release is integrated into the lever instead of the brake caliper... or use some other type of in-line cable tension adjuster.

Bottom Line: V-brakes and 5.6:1 road brake levers don't play nice together and really do require the use of TA.