There is no reason that they cannot be balanced. But most tire companies will not do it because of liability.
I used five golf balls in my bead-locks and it worked very well, you can also use BB's, air soft pellets or there is a powder out there just for balancing tires.
If you get weld-on bead-locks, make sure to spend the extra money and get the anti-coning type. Take your time in welding them on and expect some leaks, which can be a bummer to chase down.
