The only way you can diagnose this problem is to do exactly what you don't want to do. Remove the manifold assembly and check the condition of the port. If there is oil in the port, it could very well be a valve guide seal, and somehow oil is able to seep between the head and manifold (very iffy).

This is true *as long as* all external topside oil leaks are eliminated AND you are sure the head casting is not defective.

Seems like you're going to have to tear into it one way or another... don't worry, we're here! If you need to bounce some ideas in mid-teardown, I'm sure everyone will be willing to offer some advice!

-Joe