There are 2 coils on an AC system, the evaporator & condenser. You absolutely can use the either coil as a radiator, the only difference between an AC coil and car radiator is that the AC coil is built to withstand higher pressures. BUT there are a couple of concerns before you do that;
- the system is pressurized, don't just cut into it until you absolutely determine that the system has lost charge or has been evacuated.
- You shouldn't just vent R134a to atmosphere, it does have some GWP
- It's OK to mix metals (aluminum, copper) in an AC system because R134a is a dielectric but water is not and a mixed metals system will set up a galvanic battery eventually eating a hole in the system.
- you should check for pressure drop across the evaporator with water, AC system disperse a high pressure mixture across the evaporator which has a higher viscosity than water.
I should work and probably has a lot higher performance potential than anything you can purchase on the PC side, you just have to be aware of the limits.