I reported this same problem, gosh about a year ago, with ACS 4.0.
At that time, what I pointed out was that the package system and procedural API is oriented towards mounted independent packages, but offered little support for packages that have a dependency on other packages, or might want to change their behavior if another package has been mounted.
As you said, you could have a package key, but couldn't precisely determine the url (or the proper instance) of the package without diving into the SQL. How then, can one package create redirects to another package or more precisely, to the right instance of the right package?
I felt then, that the SQL solution was overly complex, introduced unneccessary complexity and maintainance constraints into the code, and while it would work, it missed the point: to interoperate reliably, and easily, packages need a richer runtime exploration API.
I ask the current architects to look at this anew.
