There is no reason why a PCI-to-PCMCIA adapter with a Verizon EVDO card wouldn't work.
I believe there are people who have done the same thing, although using the integrated PCMCIA in the MIII10000 motherboards.
If you're using a touchscreen that's designed for PC use (like the ones in the mp3car.com store), your idea about the laptop would work just fine.
If you're talking about a touchscreen integrated into an all-in-one headunit, then that's not going to work, as that touchscreen will only control the headunit.
Someone did hack a Kenwood headunit and get the touchscreen working on a PC. However, I wouldn't want to crack open a $1500 headunit jsut to pull this off...