Hi Torsten and welcome to the forum. I'm not sure what seat you have or exactly what you mean by having something solid to lean on. Perhaps you are referring to the need for more back support?
IMO, the '07 to '09 seatback is the least supportive of all 3 that Hobie has used in recent years. The '10 is a nice improvement in that regard. Shown back-to-back here, the '10 (right side) has a higher back and higher strap mounts for firmer back support.
Both have a lumbar support, but the '10 version is deflatable. Personally, I prefer the straight back and the '10 gives that option.
On the other hand, the '06 seatback offers the most support of the three. Again, compared with the '07 - '09 in this next picture (shown face to face), it is taller, with higher strap mounts and wider wings
You can see it has less padding, so the fiberglass staves support the back more directly. It also is the best breathing of any of the seats for less back sweating. There was an aluminum stave version of this seat that unfortunately could be easily deformed and became uncomfortable. This seat (with the correct staves) is still available from Hobie, and probably at a great price. This offers the best back support and is my favorite seat by far -- I even have spares for when the current one wears out! Because it is thinner, it also gives a little more effective leg length for tall pedalers who are otherwise maxed out on pedal distance.
Moving beyond back support, the i-comfort seat pad is a tremendous improvement over any of the standard seat bottoms. When sailing the AI, it isn't as noticeable since you're moving around quite a bit, although it doesn't fold under your butt like the standard pad does. But after pedaling 1/2 hour, its comfort begins to really be noticed. Shown here with my '06 seatback, I never go anywhere without it.
Finally, moving beyond comfort to convenience, here is a pic of the three backpacks that come with each of these seats.
The '06 version has loads of space, two compartments and holds cameras, lunch, and a wind breaker. Additionally, it has 2 bottle holders and has easy access to both compartments. Below it is the '07/'09 version -- much more chic and stylish, it at least holds a set of keys after unzipping it - twice (once to expand it, once to open it). Finally, the '10 version is so petite as to be virtually invisible and holds nothing at all. It is designed for the ultimate weight reduction and clean lines. Both the top two are detachable and usable as a fanny pack.
Hopes this helps address your back support issue and other matters that might be considered relevant to Hobie seating.