Weak hand crossdraw would probably be pretty good for seated draw, but has it's drawbacks the rest of the time. When I am traveling I usually carry a Lightweight Commander in a Milt Sparks Summer Special inside the pants holster. I can still draw it seated in my car, but one needs to remember to pull your long tailed shirt, or jacket tail out from under the seatbelt, or you will have a hard time getting to it.
For me if I want a holster that I can draw with either hand, then I have found that the Galco Miami Vice horizontal shoulder holster works the best, seated or whatever for me. But then I also usually keep a Glock 22 in a soft case, with the zipper unzipped, handy, so I really don't have to worry about a holstered gun, while seated in the car. At any rate awareness is the key, to having the time needed to react.