What kinds of pins are available on the Mini control board? Do we have serial/I2C/SPI available? If it’s just digital GPIO, how many?
To run an extruder, you need to be able to control the heater, heat block fan, part cooling fan, and stepper. The heat block fan could be connected to the heater, so 1 PWM line for the part fan, one digital pin for the heater and its fan, and one digital for the stepper driver. It would probably be fine to run the part fan from the existing fan line, saving one pin if needed. Forgot about the thermistor, so we need an analog input as well.
If I’m right about the requirements, the PCB to add on wouldn’t cost much. It’s a couple FETs and a stepper driver. The existing controller could probably do the work. I suspect the code needed is already available, just disabled via #ifdef.
This also assumes the power supply is up to the task. I have no idea what the current requirements are for the heater and such or what the power supply in the mini is rated for. A quick Google says the Hexagon is 12V 30W, I think Lulz uses a 24V setup. So if the wattage is the same, <1.5A, adding a little in for misc losses. Not bad.
Physically, one would need to design a dual mount that fits the mini setup and is rather compact. It needs to be reasonably light too, or deflection on the rails can become a problem. Most of the weight is in the steppers, so I’m not sure how much could be done about it.
It might be tricky to handle the auto-leveling with dual nozzles as well. Probe twice for dual prints, one for each nozzle?