But judging by how much zinc is excreted, I think it doesn't bind tightly enough to effect DMSA's mercury chelating abilities. Mercury would probably easily displace any zinc that is bound to DMSA since the bond is weak.

If DMSA bound tightly to zinc, the body would have some severe side effects to DMSA since most of the zinc in the blood would be made unavailable to the body, and zinc is crucial in so many processes. I really don't think you have to worry about zinc in regards to chelation.