This will effect the volume of hot water your tank produces though, meaning your tenants will run out of hot water faster.Another option is to have a plumber install a thermostatic shower valve.

One, maybe two thermostats (who gets to determine how warm to keep the units?).

A zoned furnace to me would mean the building has 1 furnace and each unit has its own thermostat that controls each individual apartment.  

It makes the unit a little less attractive but it keeps the tenants from doing stuff like opening windows because they get to hot, or leaving the thermostat at 76 ALL DAY. 

I get a 4 foot, 120 volt, 1kw unit with thermostat per room for well under $100/each. 

Joshua, Attic fans are simply a fan with a thermostat that cycles it on when the attic temperature reaches a specified temperature.

Make sure you have control of the thermostat or your tenants will turn it up to 90 all winter since they aren’t paying for it.

More reference: http://www.appliancepartspros.com/questions-for-ge-oven-thermostat-assembly-wb20k8-ap2623073.html

I am trying to find a thermostat with a pin code that needs to be entered in order to adjust the temp.

One advantage is each room has its own thermostat so you can turn a room off if you arent in it (ie bathroom)