Yes they can be filled but you may notice a stiffer ride since the fluid fill slows the tire flex some. Air compresses while water has to be moved to compress the air in the tire. Smaller air volume equals greater pressure change with tire movement.
(2) 2021 John Deere 6105E, 2015 John Deere 6115D with H260 loader, John Deere 5100E, 2007 JD 5403 2wd, 2008 JD 5403 MFWD with Koyker 185 loader, 2010 JD 5203 2wd, Farmall Super A, Cat 249D skid steer, 1964 JD 4020 diesel
Most ag tire manufacturers do not recommend liquid ballast in radial tires. I have two tractors with radials and liquid ballast, and one tractor with radials and wheel weights. All other tractors have bias rears with liquid ballast. I do not like liquid ballast in rear radials, it negates some of the positives of radials and has a very bouncy ride on the road, I suppose due to softer sidewalls and liquid movement.