Our old Galant had a spin-on external filter. I wish all tranny filters were that way ...
Allison/GM has an interesting concept that I liked. They have a spin-on filter for the 1000 series trans, and it only serves the valve body flow (essentially protecting the solenoid spools, etc). The rest of the trans is served by a traditional chunk-catcher flat-media filter that is only serviced upon tear-down. But the spin-on is done every 25k miles or more, depending upon service factor. The only part of the trans that really need to be "clean" is the valve body; the rest is just a giant pump and meshing gears.