First question is do you honestly need it? Sure, it sounds nice, but the units seem to live long health lives without needing any cooling or filtration.
Filtration would be nice, but what is it really going to do? You shouldn't really have any wear products (unlike an auto where the clutches wear and make junk in the fluid) and if you do have enough of a wear problem to contaminate the fluid I suspect the component wearing is going to cause problems anyway. When do a rebuild you replace most all the wear parts as a matter of course, so IMHO there's not much point.
Likewise I don't think cooling is a major problem. I'd be curious if you have actual temp numbers from your truck, but I doubt it gets over 150-180 much at all. If anything your idea would probably heat the trans fluid more than cool it. This might actually be helpful to a small degree, but not something worth the effort IMHO.
On the flip side there is a cost in time and money. Further, you add the chance for failure causing fluid contamination or loss of fluid, either one of which is likely to cause severe damage and may not be caught before destroying the trans. Ie - a line ruptures and you pump the trans dry or cooler leaks and gets water in fluid - in the former case if it happens on the freeway you could smoke the trans in short order, and in the later you'll only know if you check the fluid/coolant or it's bad enough to overflow.
IMHO minimal benefit and moderate risk - I wouldn't.
I also am unsure how difficult it would be to find an electric pump that will handle ATF. You'd also want some sort of control system.
I've looked at using a completely external water loop cooler where you'd have a separate radiator, 12v water pump and a heat exchanger on the outside of the trans case - basically the same setup as a water to air intercooler but for oil cooling. Advantage of this is is simple and commonly available parts, and there's no potential for damage is the system fails. Since the case/cover/oil pan is highly conductive you wouldn't need much contact area - I think a section of copper piping well clamped to the trans would be enough to move quite a bit of heat. Unless there's something wrong, the trans/t-case/rear end really don't generate that much heat, so there's not that much to have to remove.