Howdy everyone, new user here.
I do not have any specific knowledge of pacemakers but I do have electrical engineering experience. In my opinion the risk of harm from things like AC leakage, RF or magnetic interference, etc, from Modular is extremely low compared to the average home appliance. AC leakage, when present, is a percentage of the working voltage of a circuit. The voltages in modular are very low, no more than 15 volts and commonly only +/- 5V. So even if there were some leakage the amount of voltage to start with is so low that I doubt there would be an appreciable risk. You being around something as mundane as the 120V AC wiring in the walls or an extension cord exposes you to stronger magnetic fields than a modular rack running on 12V DC and pushing tiny audio signals around. People play electric guitars with pacemakers, and that involves holding the strings and the pickups right next to your chest while you touch the metal strings and that is generally regarded as safe.
The best way to reduce the risk of any sort of leakage current would be to use a metal case and make sure the case is grounded. The location of the power supply (internal or external) shouldn't matter so long as it is properly grounded. And as a general rule, don't cheap out on the power supply.
The one exception you might want to be careful about are modules involving tubes. Some tubes involve quite high voltages, sometimes a few hundred volts, and that is obviously a greater risk.