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.
I think I went through a similar thought process at one point: the way I think of it is like a set of train tracks. The train enters at the left and reaches a fork or split, giving it the option of the upper or lower path to the right. In the glyph, the tracks are set for the lower option.
Shift does have its own glyph: ⇧, which can be remembered as pushing the case of the typed letter up.
Best Answer
You can use Karabiner to remap keys on your keyboard.