OK, I stand corrected. But the reason why is that the LIBS are included using Require_Once(). From the PHP docs....
require_once() should be used in cases where the same file might be included and evaluated more than once during a particular execution of a script, and you want to be sure that it is included exactly once to avoid problems with function redefinitions, variable value reassignments, etc.
So the decison has to be made if we want to change Require_Once() to Require() when bringing in these resources. That will enable quick enablement of function modifications. But I need to test more to see if it wacks out something else.
And to correct my earlier correction If we can get this to work the end statement should read:include($CFG->dirroot.'\lib\weblib-custom.php');