How about just doing direct feedback on the laser position with a camera?

The Peachy team has shown videos of the Peachy illuminating the resin as it prints. Point two webcams into the container and (once calibrated) you can easily pull out both the absolute height of the resin in the container and the position that the laser's hitting it. Worst-case is that this has to go through the PC, so there could be a bit of lag (although that can be handled). Best-case is that they actually used one of the STM32 microcontrollers with a camera port on it or USB host, but that's probably pushing our luck.