My wife has family in Vegas, Palm Springs, and San Diego and many years we've driven across that area, but usually with a rental car from the airport. Great roads. I think we usually end up on 78/79 at some point (Ramona area or Julian to Ocotillo Wells) heading to/from Phoenix to visit my daughter. Sounds like a 'mostly fun' adventure. I would suspect the transmission cooling to be the issue rather than the seal on the pan. The Tip's have a liquid to liquid heat exchanger much like the engine oil cooler. It is mounted on the right (passenger side) of the transmission. It is fed coolant from the front through a vacuum actuated valve and transmission fluid through a manifold by rigid hoses that are sealed with o-rings.
Here's a pic from an
eBay sale that shows a Boxster S Tiptronic. The cooling should be pretty much the same:
It is probably not going to be easy to access, but may just need a couple o-rings or a cooler.