Yes, my first reply is a little off. I did say "without changing much of the design". You can do this with either top plates and stepped rafter seats via principle rafters or post to rafter connections where the top plate is dropped, connecting to each post below the normal plate location, CG,s or Connecting Girts. No long tenon going through a plate and into a rafter.
Have you looked at this publication?
http://www.ncptt.nps.gov/wp-content/uploads/2004-08.pdfRafters on page 37 A joint your looking for on pg 42 fig. 21
