Here's an interesting pet car seat with tether alternative. It's a rear-facing unit called "PupSaver." Manufacturer claims the smaller products may be suitable for use in the front passenger seat.
PupSaver Crash-Tested Car Safety Seat for Dogs up to 45 lbs. Air Version too!
Link below to manufacturer's comments about using the product in the front seat.
Why the PupSaver is Safe for Front Seat Use