The product is called Penray, It is a transmission sealer that you apparently add to the fluid.
Bars leak also offers two different products that are advertised by them as being remedies for transmission leaks.
Mr. Sony, question to you about all this is, can the pan rail on a transmission warp? I know head gasket surfaces can deform from too much heat or poor torque wrench work; had that happen to a friend's inline Dodge motor, supposedly built by a reputable company but ending up with me under the hood of the truck yanking the head cause it had warped. Not the same thing at all as a t-box pan rail. but where dissimilar metals come in contact, the iron and the sheet metal, with the gasket in between, there is room for issues to occur. The other thing here is that the picture is sort of upside down but I do believe that I am looking at a TH350 case and pan, that right?
The one thing that kicks my memory about the 350 is that the front pan rail is the thinnest one of the four, and very narrow right in the middle, with only the four bolts to hold the pan down and a lot of nothing in between each of them. You might very well have put your finger on the cure; that rigid aluminum pan. Any chance of scoring one at the next swap meet or on line.?
The other thing is that the Felpro PTFE blue gaskets will squirm or try to move, particularly in spots where there are only a few bolts to keep them on the path of the straight and narrow. My own experience with them is that it was miserable trying to keep the gasket centered on that front case flange, particularly right under the front pump cover, because there is so little surface there to work with. That, to me, says that the ATF is showing up in the location you highlighted but could be coming somewhere else along that gasket face and taking advantage of the gasket to weep out at the point you found.
BE aware that I am speculating here but I would not feel happy about you losing the transmission to a low fluid burnout; that is $$$ and way too much frustration; I'd be throwing things at that point and I normally do not throw stuff because then I have to go and find it.
Nick