The futon was still a bit tough to slide, so I took some inspiration from rolling step-stools, and fashioned this mechanism from a corner bracket, a caster with a threaded bolt and a spring.

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I made four of them and attached them to the futon:

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You can see the futon leaning against the interior wall in this picture as it is holding that wall to a spar while some glue drys. What you can't see is a felt slider pounded into the very end of the same board the wheels are attached to - under the recliner stops.
It is definitely an improvement, the wife can slide and operate the futon by herself, but the mechanism is not perfect, rather than going up under a load the caster posts tend to go out at an angle. I'm hoping some stiffer springs will remedy this.
Here is the back view of the front wall being attached:

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