Radiator seal type additives work by getting stuck in the leak path. A sealer that uses this principle would not work with an auto trans as it would affect the valve body.

There is a radiator stop leak that isn't harmful, is stealth, actually works on seepy leaks. It is Bars Leaks Tablets, these are also sold under the GM / AC Delco name and probably other car makers. Consisting mostly of Ginger they won't clog a heater core that is in any decent condition. Crush the tablets into powder, install the cap and drive the car.