I found my lap belt holes in the floor............................next to the seat. I don't thing they are accessible from the undercarriage seeing as how they are bolted down from the inside.
BTW, sorry for the SA remark up top, i couldn't resist
The only lapbelt hardpoints that will be available from under the car will be the ones in the transmission tunnel. They are hard to miss from underneath as they are large stampings (~2x3") that are spot welded to the tunnel and they have the threaded bosses in them for the bolts to secure the belts. The other hardpoints are in the inner rocker panel.
I've pointed them out in the attached picture. The one's on either side of the transmission tunnel are not back to back however, they are slightly off-set.
I think it would be easier to find them from under the car and push a thin nail through from below.