Extending a trip beyond your rail pass boundries?

I'm spending enough time travelling between Osaka and Yamaguchi that the JR West Setouchi Area pass looks like a decent deal (cheaper than individual tickets and includes reservations and nozomi).

My question is, if on my final day I need to start my journey on this pass and end it outside the boundaries of the pass (Hiroshima to Tokyo), can I book it as a single trip and just pay for the Osaka to Tokyo portion during the reservation? Or do I need to book it as two separate reservations?

by groggygirl