I don't know man, while I still question whether the trans is even the problem in the first place, I'm thinking that installing an external trans-cooler would be a lot less radically challenging as far as installations go than I imagine the entire process of designing, constructing, and installing a ducting mechanism for the undercarriage of a VX would be.
I've installed external trans-coolers before (even have one on my car right now that I installed years ago), so I'm aware of how relatively easy those installations are. What you are proposing is a whole other ball of wax.