Be careful with the thicker silicone based concoctions... not so much with primitive transmissions, but more-so with motors. Chunks of silicone can be squeezed inside and roam around causing problems. I'm surprised the teflon/dope solution didn't work for you. Have you tried cleaning up the threads? Also inspect around the hole closely. If someone has been wrapping too much material in there, if may be causing cracking around the drain hole. Normally for nasty ones I use one coat of dope... a couple wraps of teflon... then apply more dope and "usually" that fixes it. If you can get access to some small taps/dies for NPT maybe try lightly cleaning the threads. With newer automotive oil pan drain plugs, you can also get oversized plugs but I doubt you would get one in that style of plug/thread. Might be worth checking out if the problem persists, but again, be careful because going too big can split a casting or pan.