I'm building a garage with a second floor.  The garage floor is block, with about a 10' height.  The whole thing is 24x36, garage doors on the narrow end.  I'm planning on using open web floor trusses for the second floor, on the top of the block wall.  I want the clear span and the ability to easily route HVAC, etc. through the open web.  Anyway, instead of regular stick construction, I'm thinking about doing all or part of the the walls and roof of the second floor in post and beam - can I use timber posts with the floor trusses?

Only with solid full width blocking under each point load.

Yes, if you extend the posts past the trusses to set on the sill plates on the concrete block walls. The concrete blocks will need to be grouted in these areas, if you are not grouting the entire walls. For post and beam construction, you must transfer the point loads of the posts all the way to your footers, which, depending on the overall loading, may need to be enlarged at these areas of point loading. No type of floor truss system, I joist system, or old fashioned platform 2x floor system will carry the point loads without reinforcement in these areas.