Meaning of the Term "Heavy Metals" in CofA?
You are right heavy metals (as Pb) is a vague term. It is being discouraged and individual analysis should be reported. When someone says &quot;I tried to find a clear standard describing this to no avail&quot;, it is a good illustration that not all information is on the webpages. It is worthwhile to search books :-)
I used to think this was an old-school analytical terminology and it turns out to be correct because it is based on classical methods. From the Food Chemicals Codex (ISBN 0-309-08866-6), which is a 1000 paged book, has the following information. Later they describe the tests as well.
Heavy Metals (as Pb)

This test is designed to limit the content of common metallic
impurities colored by sulfide ion (Ag, As, Bi, Cd, Cu, Hg, Pb, Sb, Sn)
by comparing the color with a standard containing lead
(Pb) ion under the specified test conditions. It demonstrates that the
test substance is not grossly contaminated by such heavy metals, and
within the precision of the test, that it does not exceed the Heavy
Metals limit given as determined by concomitant visual comparison with
a control solution. In the specified pH range, the optimum
concentration of lead (Pb) ion for matching purposes by this method is
20 micrograms in 50 mL of solution. The most common limitation of the
Heavy Metals Test is that the color the sulfide ion produces in the
Sample Solution depends on the metals present and may not match the
color in the Lead Solution used for matching purposes. Lead sulfide is
brown, as are Ag, Bi, Cu, Hg, and Sn sulfides. While it is possible
that ions not mentioned here may also yield nonmatching colors, among
the nine common metallic impurities listed above, the sulfides with
different colors are those of As and Cd, which are yellow, and that of
Sb, which is orange.
If a yellow or orange color is observed, the following action is
indicated: Because this monograph does not include an arsenic
requirement, As should be determined. Any As found should not exceed 3
mg/kg. If these criteria are met, Cd may be a contributor to the
yellow color, so the Cd content should be determined. If an orange
color is observed, the Sb content should be determined. These
additional tests are in accord with the section on Trace Impurities in
the General Provisions of this book, as follows: ‘‘if other possible
impurities may be present, additional tests may be required, and
should be applied, as necessary, by the manufacturer, vendor, or user
to demonstrate that the substance is suitable for its intended
application.’’