I proposed another method which is similar but allows cutomization per package instance if necessary.
Basically you create a custom directory under your openacs install and can put any custom pages there in the same path as under packages. ie: custom/forms/www/index.adp
This is better if you use CVS since you don't overwrite your changes.
You an even do custom/www/package-instance/index.adp to customize per instance.
This would require a small change to the request processor. One interesting twist would be to use the regular tcl files under packages, but find an adp under /custom if it was there.