I tried filling some tubes with spray foam and testing that.
Never did post definitive results, and it made running wires through a real pain.
Off the top, it did improve the flex of the tubes, but didn’t damp vibration much, was very light for extra mass. If you just need a very little extra, maybe it would help, but in general, more trouble than it’s worth.
Increase rigidity cheaply, street racer trick?
I know it produces measureable results in a unibody car chassis, which has relatively thin metal rails, but the metal to void ratio of steel tubes combined with the more ideal shape of round tube makes it a questionable upgrade at best, and a waste of time and money being more likely.