No it will not leak any thing into the tank. LFS's use it all the time, I have used it for a while now, and I know people who has as well and have not heard of any problems.
ChrisP- planting is planting, whether done by hand or forceps or feet, if the plant is far enough under the substrate, it should stay. Tweezers/forceps make it easier to plant smaller/lighter plants and in tight spaces but many times if it goes under, it will stay.
Also I have never planted hornwort, just floated it. Though I haven't ever used it as a scape as well so I'm not sure.