What do you mean by "site-wide spamming of the toolbar"? Do you mean that you wish to display the toolbar on pages other than ec pages? And you plan to do that by placing <include src=".../toolbar>" in the master template?
I still think that it would be more straightforward to keep an <include> in the ec adp pages. In other words, don't replace the existing includes with a property tag, but place the property tag in the include, and pass it on to higher-level templates. Maybe replace the "src=..." attribute of the existing <include> tag to bring in a simple file that just sets a property to indicate the request is coming from an ec page.
The [ad_conn package_id] is the object_id of the package instance mounted on the site-map at the requested location. You can obtain various properties of any site-node with the site_node::get* api. You could use [ad_conn package_id] or [ad_conn location] as the seed to obtain everything there is to know about any node, including the package_key (i.e. the package name).