Our THC just compares set voltage to tip voltage and if tip voltage exceeds set voltage + 10%, it goes into anti-dive mode and suspends Z motion. This all happens inside the THC and has nothing to do with Mach or feedrate. If the speed slows, the lerf gets wider and the tip voltage increases. If it increases to 10% greater that set voltage, anti-dive goes active and prevents the torch from crashing. If the tip voltage comes back within range inside of 5 seconds anti-dive turns off and Z motion resumes. If not, it shuts the torch down. Aside from the fact that your speed slows without reducing cutting current, which may not produce the best cut, the system does do a good job of preventing a torch crash.
I'm guessing that by saying that his THC is "local" that means he built it himself. If that's the case, he should be able to make a few mods to the THC and get it working properly. The problem is really a THC issue, not a Mach3 issue. IMO, you shouldn't be turning off THC mode in Mach at all. If you pierce with THC mode off, you stand a good chance of beginning motion before you have a good arc transfer.