I love this thread!
 

But no other dragon other than Smaug has ever done this. I'm scurrying through my LotR books here, and I can't find anywhere else that it says that.

But it is probable the others did too, even though it isn't mentioned. Generalising is often dangerous, but at this time I believe we can take it granted for all dragons. In Roverandom, if I recall correctly, the Dragon of the Moon also did breathe green flames, even though it isn't a M-E story, it shows that Tolkien was quite fond of green fire so we could expect it from other dragons in his works as well. (Personally, I think it is some sort of expression from Tolkien to give "fairytale" quality to a "normal" fire - I wouldn't put it past him - and in that case, our question is solved: of course it was a "magic" fire.)

Túrin has some information regarding the spirit and the body of the dragon:

Though it's Túrin's own conclusion, if true, it would strongly suggest that the "fire" and the "hypnotizing" abilities come from the spirit, for the dragon proper is refered to as just the body (compare Túrins words: "His power is rather in the evil spirit that dwells within him").

Morgoth probably transferred subcreative power and lore to the drangon's spirit; a similar process took place with Sauron and the Witch-King:

Therefore, this wouldn't require the body of the dragon to possess an "indwelling fire" at all: the fire would be produced at the spitting. But, of course, the body would have to be strong enough to withstand the fire next to which it would stand.

Many suggested that the indwelling spirit would be lesser Maiar-like, but I found something possibly contrary in the Silmarillion:

Would this suggest that dragons die, for "long and slow" are their lives? Would this be possible being them Maiar?

Moreover dragons are born (or at least hatched), which militates against the Maia-theory.

This is why I thought they are rather similar to Eagles or Ents, about whom we know had some sort of spirits in them, yet they bred, grew and probably also died of old age. Similarly, we don't know, I believe, what kind of "spirits" dwelt inside the Werewolves - I believe it might have been the same. And in all cases, these were NOT maiar.*

Just for reference, that thing I meant about Ents is here (spoken by Manwë to Yavanna):
And we know the Ents could breed, have children, the same goes about Eagles (Gwaihir and his kin were only descendants of Thorondor; and you can check that Landroval is said to be Gwaihir's brother in RotK).

*It remains a question where is the border between animal spirit and this kind of spirit. Because I just thought that the animals, given their nature in M-E, probably also have simply some spirit "summoned from afar", though of different, "lesser" kind (reminds me of Aristotle); and now see, you have eagles, who are messengers of Valar and everything, but you have also let's say hawks, who are not that different from eagles, but they are "only" animals.

Legate, Tolkien actually states that the Eagles had no spirit:

Can we assume this is a canonical conclusion? Tolkien states it very conclusively, and it is, as far as I know, his last word on the subject.

Alright, thanks for bringing that up. Well, whatever you wish. Personally I was not using HoME, because I don't possess it. Anyway, I think the point is clear from the above, it wasn't about Eagles, but about Dragons - and if not Eagles, then use the Ents as example.

Just short about the Eagle thing.
It may be that that is said in Morgoth's Ring and Myths Transformed are later writings but as CT remarks there these passages show a lot of debate in the Professor's mind and you can't clearly say that he would have chosen eagles without fear over those from the Sil.