I doubt that it has seized, unless you have had lines removed, or missing. The compressor has its own oil charge, and internals are sealed from outside air/moisture. To check if it is seized or not, simply grab the drive hub (the part that sticks out in front of the belt pulley) and try to turn it with you hand (with the engine off!!!) It should be a little tight, but should turn freely. More than likely you have a low charge due to a refrigerant leak in one of the line seals, very common with cars this old, and the system has a low pressure cutoff switch to protect the system/compressor from pulling a vacuum and possibly pulling in moisture/non-condensibles into the system that would kill your compressor.
If you are really worried about your compressor engaging, you can disconnect the low pressure switch very easily, and that will keep any power going down to the compressor clutch. If you look at your firewall, on the left side of the engine (looking from front of car) there is a black canister that has your refrigerant lines going to it, and on top there is a two wire plug. Disconnect the plug, and you won't have to worry about your compressor engaging, as it will in defrost mode also.
Yuppers, with that unplugged you can put your control switch in any position and your A/C just wont work. You can use your defrost and vent, if you put it in normal A/C mode, it will be the same as having it on vent. In defrost mode, it will work just like a car without factory A/C. I have the same issue, as my A/C doesn't work, but when your windshield starts fogging up, just blow a little warm air on it and it will generally clear it up pretty well. The reason that the A/C runs with the defrost is that not only does the A/C cool the air, but it also removes moisture that will clear condensation a lot more efficiently. Also, if your compressor is seized, with that unplugged you have nothing to worry about. I know this too well because out of the seven cars I've had over the years, only one of them had working A/C!