Joe, I can't find a rule there that would inhibit use of weighting, mercury or otherwise, provided the gun makes weight limit.
It's not an issue with HG of coarse, and LG doesn't seem to mention. I see muzzle breaks are allowed to reduce recoil, so there seems no 'intent' to discourage recoil reduction. Only to prevent outright restraint of a gun's movement.
Maybe I'm missing it. Has it been checked somehow?
I sincerely wonder because it looks to me that anyone could also use magnetics both in bags and stocks. I feel that this action would be unfair to anyone who doesn't follow suit. Yet I can't picture current rules or enforcement of them actually preventing this.
There is a double edge to ambiguous rules in that victory can be innovated through interpretation.
There are some smart shooters out there, competitive shooters, who can't assume others won't interpret weak rules to their advantage.