You need something that flows easily while inside the tube and will not eventually deteriorate from friction against other particles and rubbing the inner tube. BB's- the copper plate rubs off then they rust. Lead shot- rubs away to dust and oxide against the tube. That's why most people use ceramic beads. I've been running 3/4 cup of 50/50 antifreeze in inner tubes for years. Liquids works just fine. Works for me.
As long as you do not have puncture or leak sounds great, but I imagine antifreeze creates a mess when it leaks, possibly hard on the wheels?