Seat belts are anchored to the frame (not sure if you knew that), so in theory if you could find a lap belt that would be enough size, you would just need to drill two holes in the frame large enough for the seat belt bolts, spaced a decent width apart, and torque the bolts down to the spec of the other seat belt bolts in the rear. Not sure if you could route the belt between the seat cushions or what (I don't have a Suzuki Wagon), you may have to get creative.
This is all a hack of course since the lap belt didn't come by default, but I love hacking cars :-)