I am framing in an existing pole-barn carport. Framing is 2x6 and designed by an architect. The architect's notes just said to ""Frame down existing header" over the two garage doors without additional details. I take it this is because the walls are not load-bearing - the goal is to just bring down the height to the height of the doors. Of course it needs to be structurally sound to support the door, and wind against the door.

Each garage door opening will have a short (less than 2 feet) section of new framed wall on one side and an existing 6x6 post on the other, so extending the header out past the edge of the door isn't possible, at least on one side. I'd like to use header hangers (Simpson HH6) on both sides of the door, without trimmer studs. The header will be nailed 2x6's on end with plywood spacers to bring it to 5-1/2" thick.

Does this sound workable and is there any reason a building inspector would object?

You are getting into the jury rigging area and might as well ask the inspector first. It is likely a judgement call on his part so make him happy by asking first rather than after the fact.