I don't understand what the problem is with having a 1-line (ie, just "<master>") package-level master.adp that every package page references via "<master src=master>".
In the default case, this is *identical* to simply calling the site-wide master template directly from any given package page (which should keep Jon happy). But it also allows the flexibility to easily do per-package customizations if necessary (see my example above - which should keep others happy).
I like your idea of a site-wide admin template, but I would again advise that there be a simple 1-line per-package admin template.
The basic idea is to standardize on a flexible templating hierarchy:
page-level template (/packages/foo/www/bar.adp)
V
package-level template (/packages/foo/www/master.adp)
v
site-level template (/www/default-master.adp)
I hope you will consider the above (or atleast let me know the error of my ways!)
