I have two laptops and I connect an external keyboard / mouse to them
via a KVM (Keyboard / Video / Mouse) switch box (the Video component
is not used). I frequently switch back and forth between the laptops,
but that involves manually pressing a switch button. My question is
this - does anyone make a KVM that can detect the position of a mouse
pointer at the edge of a display and automatically change focus based
on mouse pointer position - if so, who? The behaviour I'm looking for
is like the programs 'X2X' or 'Synergy', but I need the functionality
in an
external device. I am not looking for a KVM that requires manual
switching (e.g. push-button, hot-key or on-screen display
functionality).

If i understand you correctly, here is my solution;

if the computers are networked you can use this program called desktop
rover (http://www.neslosoftware.com)

you will want to connect your mouse and keyboard to one computer and
install dtrover on both machines, your primary machine will have the
mouse and keyboard... then when your mouse gets near the other screen
(basicly a side of the monitor you pick...) it will automaticly
switich the controls of the mouse and keyboard to that computer.
working similar to a dual monitor computer.

here is the free demo version;
http://www.neslosoftware.com/dtrover.exe

Unfortunately adding software isn't an option, otherwise I'd use
Synergy, X2X / X2VNC or some simillar program - this needs to be an
external KM or KVM switch box.

M.