I'm actually struggling to find the usefulness of this. The person needs to be in an fMRI machine, so we're not talking about some sort of unobtrusive scanner in a public area. On top of that, in cases like what you're describing which basically just classify thoughts to a binary level, then it's pretty much useless because if someone knows that they're being scanned to see if they're actively thinking about pink bunny slippers, then they're going to think about pink bunny slippers.