Get some degreaser, clean the motor very carefully with it, then powerwash it all off. Wait for motor to dry completely, then spray it with part 1 of a 2 part crack detector kit. The location of the leak will show up very quickly with the help of the spray.

Thanks

If you can get some kids powder paint (which is based on powdered chalk), that will also work to show up oil leaks.

Plain old flour works great too.

Why don't you remove the chain, degrease the chain and dry it then use some spray chain wax on it? This will eliminate the chain as being a possible source. Once the chain wax dries it won't drip off or fling off.

Thanks for the ideas and suggestions. I will give these a try when I get a chance to work on it again, probably in about a week.

OK, finally had a chance to spend some time on the bike and figured out the source of my leak - the injection oil tank. Dipped a piece of paper towel in the tank and there was the now-familiar red color... Which is kind of weird because I have added Yamalube 2S a time or two and I actually used the same paper towel method to make sure it is still green... but whatever, there must be a lot more red oil in there.
I found a thread addressing this from 2017. The guy, Doug D, I believe, capped off his overflow line and drilled a vent hole in his oil cap and said this worked great and stopped his bike from leaking. I'm not about to drill a hole in my original oil cap, but I was surprised to see that new ones are still available! So, I wouldn't have a problem getting a new one and drilling it. He said he only made a .062" hole in the center - doesn't that seem awful small?
There were also a couple solutions involving check valves but those sound a little dicey.
Any new ideas in the meantime? I've done some searching but not coming up with much else. If nothing else, glad to know it's not coming from my engine...

...or transmission.

Just don't fill the oil tank up so high. Are you filling it to the top? The vent overfull tube is up pretty high on the neck. What kind of riding are you doing? Are you sure the leak is coming out the vent tube and not the feed tube fitting or hose under the oil tank? 

Actually if you have the oil tank vent tube routed correctly and oil did happen to drip out it because you either have the oil tank to full or you rode a long wheelie or layed your bike down it should drip oil onto the chain. That's how Yamaha has it routed when it was new.  

I filled it pretty full. Planning to let it burn down for a while. It is definitely keeping the chain well oiled…
I ride it pretty easy, mostly on the street. She’s kind of a garage queen…