No worries, you're absolutely right, aesthetically speaking the middle is the best spot and is what I thought I'd go for.
I think unless you block some visibility of your clocks or have a very tall mount which would ruin the look, you're going to have serious problems being able to use a decent size tank bag.
The simplest ways though to mount in the center without having to get a bracket or something made up is to use a ram ball with a bolt attached and screw that in place of your tank bolt or one of your riser bolts, you can then use the mount that came with the zumo to connect on to the ball rather than the handlebar clamp and position it in the center.
It used to be that you had to go to
http://www.telferizer.com/ for one of these but they work out quite expensive for what they are.
However I just did a bit of searching for something I saw when I first got mine but wasn't on sale yet and I found it's available now.
Ram are now doing their own bolt on ball for handelbar clamps but unlike the Telferizer these just pass an M8 bolt straight through the ball, the ones they supply are shouldered bolts for the handlebar clamps but as long as it's M8 and allen head you could stick a std bolt through it, don't remember what the tank bolt is but if it's M8, you could just swap out the sholdered bolt for a threaded bolt to go in the tank position.
Here is the link, it's a heck of a lot cheaper than the telferizer ones
http://www.buybits.com/product/sku_6011.aspx If you have a look in the gallery there are pictures of pliptons bike showing a bll mount in both of the positions, clamps and tank, and i you search the forum for GPS or sat nav I know that he listed the bolt sizes for those two locations in a thread as well
I played around with all sorts of positions before I ordered any parts and just wasn't happy with the centre position because of the visibility and functionality disadvantages but it does look good
Once you've played around a bit you'll figure out what's best for you anyway but even if you want something a bit more custom like the pic you showed, it might be worth setting it up in the centre using one of the methods I mentioned just to get a feel for the location before getting the engineering work going so to speak.