Did you replace the broken O2 sensor as well?
The head gasket sealer is going to cause more problems then it will fix. It is very unlikely to clog an actual leak, but does a great job of clogging all the small passages in the radiator, thermostat, and heater core. RTV is also unlikely to seal a leak in that spot, as it is pressurized. Your best bet is to pull the motor out and do it right the first time. It will be faster in the long run than trying to hack a fix in there. Its probably going to cost you a weekend to do it in a driveway.