It may be hard to find a gel solution that does not interfere with back support, but I would look at the smaller gel pads made by Action Products (larger ones can be cut to size, also). You would probably need to strap it to your chair so it wouldn't fall. I think you could staple or tape straps on -- the gel is pretty robust, which is why I like those. But strapping and the pad itself creates pressure as well. You really want a back cushion that cradles your back, and nothing else between you and your body, just like with your seat cushion.
To have an extra layer over the smallest possible surface area, another idea is to cut/shape/tape on something like a gel heel or elbow protector, but that too creates pressure.
Also, you can take off the back cushion and replace it with another in some cases, if it's velcroed on. Depending on what back you have, this may or may not require your DME to order. Obviously either way paying cash expedites things.
I would also make sure you shift tilt/recline position frequently if you're feeling undue pressure, and double-check your headrest and footrest height/angles to make sure that is not the problem relative to your back pressure.
If the pressure is worse when moving, I would also look at your tires.