What is your opinion on using bar's leak to seal a leak in the heater core?

There is a little chance it will hold permanently - if it holds at all. These kind of additives has a tendency to create actually more cooling system problems. I have seen many heater valves stuck, blocked coolant passages, etc.. Personally I'd never use stop-leaks in my cars because of the reasons above. Zee

Replacing the heater core will in the long run be cheaper than using bars leak. Bar-leak will typically seal "holes" that are supposed to be there, like the actual tubes of the heater core the heater control valve, tubes in the radiator, passages in the intake manifold........ which will just make the eventual real repair cost more and more.