I think the ideal way to go is with a pump and external heat exchanger. Trans/diff pumps exist, but they aren't cheap. There must be a cheaper alternative out there.
Pumps - Mocal Differential Cooler Pump
http://www.summitracing.com/search/p...l-cooler-pumps
Combine a pump with a plate heat exchanger.
Plate heat exchanger - Wikipedia, the free encyclopedia
You could also add a filter. And even use the pump to change the fluid!
What about using a fluid thinner than recommended and just adding a trans cooler to keep viscosity up on long trips and/or hot weather? Same result of ideal and consistent viscosity without the warmup wait?