If You do a search You will find a thread on welding anodized aluminum, there is a link to an article about Tuna Towers on boats.
My opinion is that unless You weld the joints it will look like a farmer built it.
With the mounting detail shown, You would have to use shims to get the posts plumb.
If the connecting part was an angle instead of a "Z" You could bolt through the 4" dimension of the post and locate the holes so that the posts are plumb even if the mountings are not. This would allow enough freedom that You could fabricate as large a sections as You can handle in the shop.
The top rail may tend to warp from the post welds. You can build a simple jig with a porta power cylinder to true it up after welding. One of the marine rail builders I know does this.