Not specifically concerning to heavy metals. ― Besides classic analytical chemistry textbooks that cover complexometry, the signalled book references also may help to identify suitable masking/demasking agents or procedures:
D.D. Perrin, "Masking and Demasking of Chemical Reactions: Theoretical Aspects and Practical Applications (Chemical Analysis)", Series: Chemical Analysis (vol. 33), Wiley-Interscience, New York, 1970.
Eugene W. Berg, "Physical and Chemical Methods of Separation", McGraw-Hill, New York (etc.), 1963, sections 18.3 & 18.4.
Some lead(II) chelating agents that you may want to consider: citrate, dimercaprol (BAL), 2,3-dimercaptopropanol, ethylenediaminetetraacetic acid (EDTA), D-penicillamine, or tartrate.