The original PCB is symmetric, so that would be doable with custom firmware. I think a few people throughout the years have posted that they hade built them, or at least considered to.
But you'd need to build it like a right hand but mirrored, with the components on the opposite site, including the Teensy 2.0 upside-down ... That is if you can get hold of a Teensy 2.0. The original maker: PJRC, has discontinued it and are out of stock. Other stores may have stock left. There might also be a clone of the Teensy 2.0 out there.
None of the ARM-based Teensies from PJRC's current lineup will work - they have a different pin-out, among other differences.
You might want to consider looking at other ergo keyboards too. The convention among DIY ergo-mech keyboards is still symmetric PCBs but with a
Pro Micro on
each side.
Most keyboards have even fewer keys than the ErgoDox (76-80) though.
I'd suggest looking at the Lily58 (58 keys) or the ErgoDash (66-70 keys)
Here's a site that lists lots of keyboards that lets you compare the layouts:
https://jhelvy.shinyapps.io/splitkbcompare/