Can tire sealant be used in anti-decubitus cushion?
I'll do my best to help with my knowledge of sealant, and this was too long to comment. 

Sealant MIGHT work.... but it may cause more issues than it fixes. 

My main concern would be getting it from cell to cell, as in a tire or tube it is easily distributed along the entire tire/tube, whereas this look would take some work to get it all the way to the other corner from that of the valve. Also it depends on the size of the hole or puncture as sealant is made for relatively small holes. (1/4" to 1/8" depending on brand) 

Also it may impede air from getting to some of the cells if it gets clogged up, it's hard to tell how large the airways between the cells are from the image. 

The other kicker is sealant does not last forever, in a tire or a disposable tube it can be removed if it begins to clump or harden, or the tube can be easily and cost effectively replaced. Where as this, you have said is semi-expensive so if the sealant clogs some of the airways and cells no longer inflate you are kind of stuck with a defective cushion that will then put more pressure on certain areas.

Further more there is not much to get it to the top of the cells, so it will collect in the lower regions, and if there is a puncture near the top of a cell, may or may not do anything. 

If you have an older one you can try it on without losing too much i would try that. Otherwise it may cause more frustration than it fixes. Hope this helps.