Actually, those would be categorized as "self-sealing" tires.
Real "run-flat" tires can be aired down to zero psi and still be driven on.
You are probably referring to the green "slime" stuff. It would be an interesting idea, probably wouldn't hurt unless it plugged up a TPMS system you were using.
I had "self-sealing" tires from the factory on a 2002 Chevy Venture we owned. They were made by General Tire. I don't hear much about self-sealing tires lately, don't know if they are still around.
Now, real run-flat tires for a trailer would be interesting.
Steve