After knowing that some North American 32w circline floor lamps and preheat CFL adapters use toroidal preheat ballasts, I am beginning to wonder if there were any HID fixtures that used toroidal ballasts.

First a question: Were those fluorescent ballasts really toroidal, or it were just some flat ballast in a round housing?

The thing is, making things requiring an air gap in its magnetic circuit is quite impractical in a toroidal form.
Standard transformers are made by winding the strip of magnetic steel sheet to form the core, then wind an insulation/protective tape (to separate the copper wire from the core) all the electrical windings all around on it.
Once you need a gap, you need to either cut the wound core and then assemble it back with the gap forming spacer, but then you need some armature to keep the halves tight together. And those armatures will then form obstructions for the main winding, so making the electrical coil gets more complicated.
Or you need to make the core out of a material with the air gap distributed within its mass, so e.g. an iron powder or so. There could be some other construction methods, but they all would have one thing in common: Be way more expensive. Even the flat round enclosure optimized version of a classical ballast style would be more expensive than when there is no such space constrain (iron and copper usage,...)
So I would guess such format would be avoided unless there is really no other way. And as I may understand with small fluorescent lanterbs, there is a desire to use compact round ballast enclosure, I don't think there is such need for the HID's to .

What I've seen inside those flat round ballasts were really flat rectangular ballasts (talking about the "TU" format common in Europe), with dimensions set so it quite well fills the round case they are housed in. To me it looks like there is no way a toroidal format would be any more effective in utilizing the round space than these, rather standard style formats.