I just wanted to share, that it is in fact possible to control Bluetooth Mesh lights via MQTT and Home Assistant. I use a python library on top of the Bluez bluetooth-mesh service to retrieve device information, turn devices on/off, set the brightness and light color.
My current state is available here GitHub - dominikberse/homeassistant-bluetooth-mesh: Allows to use bluetooth mesh devices directly from homeassistant, but it is still very much under development and might be a little tricky to configure and set up.