Digressing on the G-ETP idea a bit further..
..instead of copying the .tcl from the package-key/www, a tcl file in the serverroot/www tree could probably (untested) source the original, and include new variable calculations etc so that most changes via apm will automatically update the tcl, too, while retaining customized tcl code.
# foo.tcl
# calculations before sourcing original file (if any)
# source original file
source /var/lib/aolserver/service1/packages/package-key/www/foo.tcl
# calculations after sourcing original file
