Not sure about this reasoning. When the computer is in sleep mode, the hard drive also stops spinning. I replaced my 3.5" HD with a 2.5" PowerBook hard drive and found the interior of mine much cooler. Also, I took the thermostat approach and find my fan comes on mainly when the hard drive and CPU have been powered up for some time. I don't recall hearing it when just the screensaver has been on.
Can't remember if you can independently spin down the hard drive, but if you can, see what happens when you put the HD to sleep and run something only in RAM like a screen saver without putting the system to sleep. I think some later versions of System 7 allow this. If not, you should be able to unmount the HD, or temporarily unplug it an load your system from an external drive or floppy.
If you gain better heat performance, I would definitely consider going with a PowerBook drive or a flash drive instead.