I’ve been digging into the code for this, and it appears to be in lib/rc , which is only referenced in modules/px4iofirmware , and doesn’t seem to be part of the HAL now.
I did get the hott_telemetry driver to work on Serial 4 (/dev/ttyS6) with a GR-24L, but the MZ-24 doesn’t show much data from it.