No, but I've been thinking about it for a while now too.
You might be interested in this video of a guy named Gav installing one in his Citroën C-Zero / Mitsubishi i-MiEV. He has posted a lot of detailed content about his trunk-mounted install you can find here:
https://kiwiev.com/installing-a-diesel-parking-heater/
and associated video
Another video of a van EV with an engine-bay mounted air source fuel heater installed with its inlet and outlet in the footwells!
Instead of an air source heater, there are also coolant/boiler heaters.
Ben Nelson has installed such a heater in his i-MiEV.
https://300mpg.org/imiev-heater-installation/
There are some advantages to using boilers, as the heat can be used with the existing HVAC system efficiently, and in tandem with the PTC heater.
Since the Spark has 3 coolant loops, there are future possibilities worth committing to a coolant heating system, such as running connecting heat exchangers between the separate loops to potentially sharing heat with the batteries. This hold merit because the batteries want to be the same temperature people do, the heater is a whopping 2.4kW and activates regardless of whether you want it to, and regen is hard on the batteries below freezing, and is limited a bit, but still allowed, causing additional degredation!
