Yes, you can go through the wall below ground level.
Any good non-hardening caulk should give you a seal.

If you are running 120 V to the lamp, watch the cover rules for buying cable or conduit.

In many cases it is actually faster to use intermediate or rigid metal conduit since the cover depth is only 6 inches.

You can easily run a shallow trench with a spade, and even put the 'plugs' of grass right back in place.

I am curious about this as well, and have one additional question. What if you wanted to wire sentries on both sides at the end of the driveway? What's the best method to get the cabling underneath the driveway and is that even legal?

Thanks for you replies. In my area, I can go 18" deep if I use schedule 80 PVC, so that's what I intend to run. To seal the hole, what would be best? The grey conduit sealant, or something like PL polyurethane sealant? Thanks.

"In my area, I can go 18" deep if I use schedule 80 PVC ..."

I hope it is not a long run, an 18+ inch trench is a lot of work without power equipment like a backhoe.

Remember that the top of the conduit must be 18 inches below the final surface.

For a single family dwelling and the circuit is used for dwelling-related purposes only, direct burial depth can be 12 inches if the circuit is 20 amperes or less, 120 volts or less and GFCI protected.

While you can get away with even cable 12 inches deepo, watch very caarefully were the cable is going to be run.

I just replaced a lamp post feed a few weeks ago that was cut.

12 inches is well within 'shovel depth' for turning over or putting in a garden, and that is what happened.
The plot of lilies beside the front steps was directly over the cable, and it was cut during preparing the soil.

At least with intermediate or rigid metal conduit, even at 6 inches of cover, nothing short of large power equipment is going to damage the conduit.

Thank you all for your replies. Since I'm going to use schedule 80 pvc conduit buried 18 inches deep using my neighbor's trenching machine, will the grey conduit sealing putty work underground? thanks.

I use cement listed for the conduit.

"At least with intermediate or rigid metal conduit, even at 6 inches of cover, nothing short of large power equipment is going to damage the conduit"

Brick - does the rigid metal ever rust? I have seen this in home centers it appears to may be lightly zinc coated or galvanzied. Not sure.

"does the rigid metal ever rust? I have seen this in home centers it appears to may be lightly zinc coated or galvanzied. Not sure."

It is so much thicker than needed it is not a problem for burial.

The wall thickness on rigid is approaching 1/4 inch.