I can only offer the mile-high perspective: plants synthesize vitamin B1, so you want to be careful about spending too much money on a supplement. Is the idea that even though hops can make their own B1, they just need a boost when they're being transplanted?
Why not use yeast slurry as a source? Yeast contain a ton of TPP (B1), so just pour off the beer, and dump a big yeast cake in the hole near the hop roots.
People also go to the health food store to buy bottles of ubiquinone (sold as Coenzyme Q10 or CoQ). Your body makes tons of this stuff, and every cell in your body would die if it didn't have enough. Still, people pop these pills expecting some benefit. I guess what I'm saying is: I wouldn't want you to spend too much on the B1 for the hops.