Interesting. I know the problem with silkworms and thiaminase has come up on some frog forums I'm on. I think the majority of the real worries with thiaminase come from the feeding of live rosy reds or feeder goldfish rather than f/t stuff since safe fish such as tilapia seem to be the more easily and cheaply acquired frozen options. I actually don't think I've ever seen someone asking about a frozen source that isn't on the safelist just because those aren't so common...

I think not much is known about the thiaminase issue, even in regards to feeding what are considered 'dangerous' species. Some people can do it for years and have no problems while others can do it once or twice and begin to see the neurological symptoms of the vitamin deficiency. Scientific debate aside, I probably wouldn't recommend even frozen thiaminase-containing fish as feeders just because there are so many options where we know for certain it isn't a risk.