Interesting. I've wanted to put headlight bulbs into my fog lights for a brighter, second set of 'eyes' on the backroads during late night runs. I wonder about heat, being such a small light housing, but they do have a vent hose, maybe for such reasons. I still haven't gotten around to it and certainly hadn't thought of putting large assemblies or ballast in and around there.
When thinking of HID retrofit, I think of the Honda S2000 units often used. So I searched that today and found this manufacturers site. Maybe they have some solutions for your wants.
https://www.theretrofitsource.com/compo ... s2000.html
Do you drive a lot in inclement weather or unlit back roads?