As a biologist, I'm well aware that thiaminase isn't a toxin and how nutritional deficiencies work.

However, I'm trying to give practical advice, not debate the molecular workings of thiaminase. The fact is we've had members on here with snakes experiencing severe neurological symptoms after only a few feedings of thiaminase-containing species. Do we know for sure that this was due to thiaminase? No. Were these snakes experiencing known symptoms of B12 deficiency after only a few feedings of a thiaminase-rich species? Yes.
In seeing over four years' worth of posts about potential thiaminase-related symptoms and deaths it really seems that it affects different individuals differently, and seems to have the greatest affect on immature animals. Some people can feed thiaminase-containing species for years with no problems. IMO it is not worth the risk and should not be done just for variety's sake.

As I said, rosy reds are certainly better than the animal starving, but IMO it's plain irresponsible to recommend feeding a species known to contain high levels of thiaminase so long as the rest of the diet contains B12, and even moreso to just recommend that a typical hobbyist try to use a B12 supplement as most people have no way of measuring the vitamin content of their snakes' food or knowing what the appropriate dosage would be. You can overdose on B12, and a B12 shot is a risky procedure for deficient garters even when administered by a vet due to their small size. Also, I'm unaware of any B12 supplement marketed for reptiles or amphibians that is available without a prescription.

Also, I haven't had time to read the papers you linked to earlier, but do they provide comparisons in thiaminase levels between simple cell breakdown due to freezing and what is present in species such as goldfish that produce high levels of it naturally? I would wonder if a low-thiaminase content species would have a comparable amount after freezing to a high-thiaminase content species prior to freezing. My gut feeling is that the content would still be lower in the frozen 'safe species'.

The point here isn't whether or not it would be hypothetically okay for the snake's diet to include rosy reds due to their thiaminase content, but to recommend the safest possible diet to another keeper.