I have not seen nor read of a technique or product to seal the cable housing to the transfer case. I do know how difficult it is to remove the threaded cap when it's put on too tight.
From my own personal experience, I haven't had any leakage from the threaded cap of the cable housing. Also, from personal experience, I've been fooled many times on where a leak originates. One technique is to add dye to the oil and then you can see where it's coming from. For me, spraying/cleaning/drying works really well and is cheaper. Once all the oil is removed, a dry paper towel dabbed in the area (start high and work lower) will show where the leak originates.
How full is your transfer case? If you remove the filler cap, does the oil: Pour out? Drip out? Not drip?
The threaded cap should screw all the way down so that there's no play in the housing (I believe THIS is the seal to the housing at the transfer case). Are the threads clean on both the cap and transfer case? Could the cap be cross threaded or damaged? Since the cable is supposed to be serviceable vs. permanent, sealing the cable housing cap may cause it to be non-removable. So, if you have issues with the cable itself, it won't be easy to fix (here I'm thinking of a broken cable down near the transfer case).
Sometimes these things can't be resolved while sitting on the jeep (our best efforts to contort and squeeze into a confined space just isn't enough - and repairs on a work bench become more desirable. If you have eliminated everything else or need to buy time for a better attempt later, you could smear some thick grease into the cable housing cap and reinstall it back onto the transfer case. It may slow down a small drip, shouldn't be incompatible with the gear oil and won't interfere with removal later on.
