Am in the same boat, and unfortunately I don't think you managed to answer the question.
The problem: I have no idea if a postroll was scheduled that I should wait for. Is there an API I can check?
What: Nonlinear overlay ad served (using IMA provided sample), no other ads.
When: Main content has ended, and I have called contentComplete()
Expected: ALL_ADS_COMPLETED to fire when after any postrolls play, and I advance the playlist.
Actual: Nothing fires, I never advance the playlist.
My workaround: If the current ad is nonlinear the main content ends, I just destroy the ad manager and skip whatever post-rolls that might have been scheduled.
Looked through the example code but it does not appear to cover this case. Is the answer that the SDK just does not support both non-linear and post-roll ads at the same time? I am still digging around to see if there is a hook into the VMAP schedule. Flash SDK has same problem, for what that's worth.
Cheers,
Greg