If you look at the cross section diagrams I posted above, that counter gear shaft rearward end should be almost flush with the case and there is no seal or "plug" to prevent leakage. I thinks it's unlikely that you will find anything that will stick to that small gap/space to prevent leakage in its present state. Unless you tear the transmission down, thoroughly clean and during reassembly apply some sort of sealant it's unlikely to stay put.
The difficulty with that is if you read the counter gear installation instructions in the shop manual, there is supposed to be a dowel like tool that is pushed into the case to capture the gear, needle bearings and counter gear trust washers (fore and aft) and hold in alignment for the counter ger shaft to be installed. Then that tool gets pushed out the rear case hole as the counter gear shaft is installed until seated fully at the front of the case.
Perhaps if when you reassemble you put a good coat of heavy grease in that hole, when the shaft settles into its appointed place the grease will reduce the amount of leakage/seepage of oil?
