Joe:
The issue here is the movement of the wood, from summer to winter or dry season to wet season.
Wood moves, your copper flashing will not move the same, in my opinion.
This leads to gaps between the wood and copper.
Rain water will find its way between the wood and copper and get down into the pocket created by placing the wood into the concrete.
A concrete cup below grade with a piece of wood in, will let in water, and the wood will rot.
This is why they have code rules to prevent builders from doing this as it does happen.
You'll need to end the wood above grade and do all the concrete and rebar stuff you want below grade.
There are many hold down systems for posts to concrete.
Look at Simpson strong ties at:
click here for link to Simpson Strong Tie site You'll find lots of things that will work and be allowed by code rules.
Jim Rogers