The collection-specific perllib/plugins directory should work fine. That is,
the greenstone building code will look there for a plugin before looking in
the general gsdl/perllib/plugins directory (so if you had an HTMLPlug.pm in
both places the collection-specific version would be used).

The collection-specific macros directory on the other hand is useless for
the general greenstone distribution. This directory is only used by
collection-specific receptionists (receptionist is the fancy term we like to
use for the greenstone run-time system --- i.e. the library cgi binary). To
create a collection-specific receptionist you need to alter the C++ code
slightly and recompile.

regards,
Stefan.