GS, that looks like what I might need. I'm thinking I will try raising the bar with an adjustable stem first. I've got one of those bad backs, too!I think an earlier thread got the ball rolling on flat bars. I give @Ron Bez for bringing this topic up.
I tried an extension but it wasn't adjustable. I think the key may be that your hands and wrists end up in a neutral position.
The distance from the seat to the handlebars (or grips) must determine how far you lean forward. My wrists and thumbs got so bad with a flat bar I built the cruiser in my member photo (left). This works, but it seems to put a lot of stress on the rider's back, it's too upright. I bought a third bike a while after building the cruiser and it came with the standard flat bar. I was in pain the first time I rode it. I got rid of it immediately.
This is the bar I ended up with, and the nice grips. I have to be careful from here. I don't want it to start looking cluttered. You do get a lot of room with some of these curved bars.
Wearing gloves and moving your hands around, avoiding the 'death grip', all help.
It's a Sony AS-100, so now discontinued. The image stabilization helps. I get the best stability with a chest mount but you really stand out doing that. The bar mount is pretty stealth.BTW, is that a Sony videocam on your bars?