The 400/800/1440K drive which is powered off of the floppy port, and it is essentially a motor that can be very active during file transfers. I've never seen a sticky drive overload the bus, so there must be safeguards in place, since common disk accidents would damage the Mac in the way you suggest and I just never saw that happen. And the port can only power a DC fan.
The serial port was never designed to run a drive, so I would be more concerned about running a motor off of that.
Plus, I'm not sure how using a port as a power source is any different than the common practice of taping into various power access points inside the Mac to power internal AC & DC fans.