I've got a 1972 AT2 that is slowly leaking oil while sitting from I believe the centerline case seal at the bottom of the engine. I really don't want to pull and split the motor. Has anyone successfully drained the oil, cleaned the exterior of the case, and resealed it externally?  Thanks 

That's not a very common place to leak in my experience.  And oil leaks can sometimes be hard to pinpoint.  

Two things I'd try is clean the bottom of the engine really good and then dust with baby powder or flour after it sits a while. (you can't ride it because oil gets blown all over when operating) If you cleaned really good, the powder will only stick where oil is leaking and will make the leak very visible.

Another thing I've done is drain the oil and pressurize the trans through the vent hose with no more than 5 psi and then spray suspected areas with soapy water and look for bubbles.

A third thing is UV dye and blacklight is very affordable these days.  Add dye to oil and the UV light will glow brightly showing you the leak.

It could just be the drain plug?

As to your original question, I have never had any luck sealing leaks externally by gooping sealer over the joint.  I've never tried it on a Yamaha but on similar situations, it always starts leaking again soon.