Short answer: I didn’t need it. I extended the platform to suit my own needs.
Having said that, you’ve provided a compelling example of why a temperature_command_template would be useful. When I have the time, I’ll toss that one into the mix and post it in this thread: Enhanced version of MQTT HVAC (Climate platform) with proper History Chart
I believe the low/high temperature control is in the Pull Request pipeline: Add target temperature low high to MQTT climate by rwagoner · Pull Request #17391 · home-assistant/core · GitHub