Can you put brake fluid in your oil tank to unlock the engine?

I have never heard of such a thing and I wouldn't recommend it. I have heard of old timers putting a teaspoon of motor oil in their tank when they filled it to preserve valves. I wouldn't recommend that either. If your motor is seized ( you said locked ) this wouldn't help. If your motor is seized you can't turn it by putting a wrench on your crankshaft bolt. This usually means something is wrong in the motor itselfand adding something to your fuel will be fruitless.

If the engine has been sitting for years and is just locked due to non-use, remove all spark plugs and pour 1/2 cup of kerosene in each cylinder. Wait 48 hours see if you can turn it over with a socket and ratchet at the crankshaft. If you can, then change the oil/filter, put the spark plugs back in and see if it will turn over.

If your motor has interference heads do not try to start it with out finding out if it is out of time. If one of these motors is out of time you won't be able to turn it because a piston(s) is touching one of the open valves. Starting it could shoot one of those valves through your valve cover or damage ( or break ) a rod.

If the vehicle has been sitting for years ( as above ) the timing belt is much more prone to breaking. The polymer in the belt conforms to the shape of the pulleys and becomes brittle. Most recommend 4 years or a certain number of miles ( ie 60K for BMW). Consider this if your motor has interference heads; even if it is in time and has been sitting you should probably change the timing belt.