I have found 2 issues which may impact this.
1. Reference to DIR_SYSTEM has been made in a file included in init.php BEFORE definition of DIR_SYSTEM occurs.
2. The PHP system() function is used in three places in the cart, including the package manager. Not all servers allow the use of the system function. In fact, some servers periodically change which functions are allowed for security reasons.
The upshot of this is, the temporary fix I mentioned above should be safe. Some users may need to replace the system() function calls with calls to exec() or other functions in that family in order to get the functionality provided. I'm not fully clear at this point on just what that is, but it would appear that this includes OR possibly includes backup/restore, data import/export and package management functions possibly including upgrades.
David