there is no such thing as "safe lead"...the lead weights that anglers use are actually part of a bit of a controversial topic...waterfowl are often found with loads of lead weights in their guts having died from the toixicity of them...there is a move to replace all lead fishing weights with less harmful/toxic metals to avoid this wildlife poisoning. it is a big deal around here (Upstate NY) as the common loon populations in the Adirondack Mountains are taking a huge hit due to lead weights.
so id stick to using a rock or something to hold down the plant. lead in any form is not good for the environment as a whole.