Thanks to jujuhtst, a member of the forums, for telling us that what we discovered two years ago is still working, replacing External by Internal still allows the use of DVD Player with Lion with an external DVD drive if the internal Superdrive has been removed. Here is what we first published about that, back in 2002 (the link is to the French Macbidouille, but the English translation is on the page):
Go to System/Library/Frameworks/DVDPlayback.framework/version/A
Copy DVDPlayback on the desktop and make another copy elsewhere.
With an Hex editor, look for all the "Internal" in the file and replace it by "External" every time it occurs (4 times).
Save the file and put it instead of the original one (you will have to type in your admin password).
From that point, DVD Player will work with an external DVD drive.
Beyond the limitation, it is all about money and patents. Apple pays licenses for each computer with a DVD drive, and of course, Apple doesn't have to pay for those that don't have one, and that is why the use of external drives is blocked.