It's 4 years later, and I owe this thread a followup with my final solution. I've had a reminder to post a followup on my bookmarks bar since July 2016 and I'm finally sitting down to write it while stuck in quarantine.
I ended up purchasing a JuiceBox from emotorwerks.com and mounting it on my house. I cut the EVSE charing cable and connected the four wires to 3 #8 AWG and 1 #16 AWG solid wires run through a PVC conduit underground. L1, L2 and ground used the heavy wire and the pilot used the thin one. We used a concrete saw to cut a slot in the sidewalk large enough for a shovel and buried the conduit 24 inches below ground level. The termination was in a NEMA 14-50 outlet. Then attached a NEMA 14-50 plug to the changing cable. We patched the sidewalk using bagged quickset concrete.
When I want to charge I plug the cable into the outlet and then into the car. When done I store the cable in my car.
I mounted the plug on the tree facing away from view as much as possible. I now "plug into the tree" to charge. I joke about planting a "current bush" next to the tree so I would be charging using that.
I understand you can tunnel under sidewalks with a garden hose attached to a piece of metal electrical conduit, using the water to blast away the dirt. I would try that next time.
Here is a cut and paste of advice I received from emotorwerks support:
From the JuiceBox, run the 4 conductors separately (of the J1772 cable) in a conduit to terminate with a 4 prong outlet of your choice on the other side of the sidewalk. Use a weatherproof enclosure. Then take the J1772 at the length you need (up to 25' as allowed per code) and wire a 4 prong plug to match your outlet, lining up the L1, L2, ground, and pilot. You should then be able to plug into the outlet wth your J1772 "plug" and charge.
Cut the cable from the J1772 close to the box to save as much of the length as you need. It might have 5 wires inside, we only use 4 of them. Tie them into a junction box. At the junction box, run out to the street (under the sidewalk with conduit and wire, #8 gauge is fine for 40A continuous and easier to work with) you can use #8AWG, THHN, 1 blk, 1 red, 1 green, and 1 #16AWG blue for pilot. Splice wire to wire in the box. At the street, terminate the other end at an electrical box into a NEMA 14-50 outlet. Use all 4 blades, and i recommend using the ground for the ground, L1 and L2 on the sides, and the neutral blade can now be your pilot. Then add a NEMA 14-50 plug onto the cut J1772 cable, and wire it the same on the plug so that the blade opening with black, is black, red to red, etc. You'll notice in the oic, I would do that, and make the W the blue pilot.