The OpenEVSE board could probably be adapted to do the job, although if you went J1772, then other vehicles would actually be supported...Tesla isn't the world after all.
Have you contacted Tesla, yet? There may be an official way to support this configuration. The worst they can say is no.
Yeah, OpenEVSE's AC detection stuff would need modification (or deletion), since it relies on the neutral/ground bond.
I'm fine with J1772 protocol, but as far as I'm concerned Tesla is all that really matters on my end, so sticking with the Tesla connectors for sure. I've made a Tesla to J1772 adapter out of an old Tesla charge ports that I use to charge Volts at my house, though.
Edit: Oh, and I did reach out to Tesla. No response yet.