Of course, this is looking at the brain's power management system and trying to infer from that what processing is going on. If you did that for a CPU, you could see the FPU or the I/O controllers heating up, get a sense of how many cache misses were going on, and see how many CPU cores were doing anything. You could probably tell a game, spreadsheet, or web browser apart. This would not lead to much insight.
It would probably lead to more insight than not looking at which parts heat up. State of the art before fMRI was looking at people with very specific brain injuries, or using EEGs, which offer even lower fidelity.