As you can see from the responses, there are no definitive answers to this question. In my Glorantha:
(1) "True" draconic magic is a by-product of Draconic mysticism. It is not something you learn; it is something you realize after having obtained a sufficient level of enlightenment. And it is not something you cast; you essentially just use it or not use it. This is probably not helpful to you since you want PCs to use it in a game.
(2) If you somehow do become an initiate of Draconic mysticism (meeting a dragon, becoming possessed by a second age ghost, being instructed by a renegade dragonewt), I have always liked the suggestion (made by Sandy Petersen in a zine?) that using dragon magic destroys your personality. For example (applying this to the current rules), using a draconic healing spell would reduce your Fertility Rune percentage, while sprout wings might reduce your Mobility Rune. A dragonewt can eventually recover from this and rebalance, while a human cannot (i.e., it reduces your maximum in a rune). So human users of dragon magic become more and more alien and probably become monsters if they destroy too much of their personality.
(3) But as mentioned in the earlier answers, there are plenty of "false" dragon magic cult that a player character can join; for example the Path of Immanent Mastery, which teach spells that look like dragon magic as ordinary rune magic or sorcery. The EWF probably had something similar (it started off as "true" Draconic mysticism, but lost its way) which could be rediscovered by an adventurer. I don't know if you have the old Gods of Glorantha book, but there are versions of the Path of Immanent Mastery available on the internet to use a model.
I hope this is helpful.