I just got a brilliant idea!

As long as you're thinking custom modifications, could you do this: Have a second set of tires on the trailor, like a tandem set-up. But, (this is the cool part), the second set of tires are a light & wide tire that are a smaller diameter than the street tire. On the highway, only the street tire is contacting the road. But in the sand, the street tire sinks a bit and allows the smaller wide tires to float on the sand to prevent further sinking.

(Royality checks, please.)