How do I change GNU Screen's meta-key to Cmd-a?
I generally think the command idea is a bad one, because it is intended to be the system key for app manipulation. Cmd-l appears to be unbound in the application menus, but I couldn't use any tricks to view what it's keycode may be. Thus, I would suggest Shift-Ctrl-a.

Having said that, if I understand the man page, and a resource on the Ubuntu Forums on the topic, I believe you can change it to Shift-Ctrl-a either by using the -e flag (which I suggest you use to test with before putting it in your ~/.screenrc).

You should be able to run screen -e ^AA. ^ is a common identifier for "Ctrl", and A means capital-a (shift + a), as opposed to a lowercase a, meaning the character by itself.

If that works, you should be able to add a line into your ~/.screenrc that says: escape ^AA for it to automatically take place on any standard execution of screen just by itself.