Yeah, poking around I have found some fragments of stuff. There's a 13th Age Glorantha book too that tries to approach heroquesting. It sounds like the gist of it is a fairly cool way of flavoring an adventure, and at least in the original game IIRC (dimly, didn't play much RQ) there were some variant magic rules and such. Sounds like the newer incarnations follow that pattern as well.
So, because the 'mythology' of Glorantha (or other settings like 13a Dragon Empire) are not codified, you can basically construct whatever narrative you want, or in Story Now terms I guess just assume that 'what happens follows the myth'. Nothing wrong with that, it is a pragmatic approach, and then you just write a lot of "and then the band of heroes..."
I had trouble getting my head around Heroquesting to be honest, but I understood it somewhat hen playing the computer game about running your gloranthan tribe (king of the dragon pass?). From the point of view of the tribe, the myth is obviously set, and they look for opportunity to re-enact the myth: trying to emulate it as well as possible, with the possibility of irritating the god if you do it badly. And you must deal with the consequences. If you re-enact the myth of Orlanth plucking the eye out of the Troll God by killing a Troll chieftain, there is strong possibility your diplomatic standing with the Troll tribe will plummet.
So, to take an illustration from a "set in stone myth", let's say one would want to gather the favor of Apollo, he could look for opportunity to do an easy, one-step quest (like re-enacting the killing of Python, just find a suitably large serpent and kill him... if you kill a dragon, you'll probably succeed your heroquest, if you only kill an adder, you'll probably incur the wrath of the god) and you get a small boon. More complex quests are multistep. For example, when going with the myth of Perseus, where he's born and cast at sea with his mother, then land somewhere where he is raised and ends up killing the local king, and becomes known for tricking a group of 3 hags to get a few magical items, and then defeating Medusa... A
known heroquest would be, as a multi-step quest:
1. Embark at sea and let you derive, landing somewhere after surviving the trip
2. Steal a precious possession from a coven of hags (their hag eye? The soul bag of a night hag?)
3. Kill a Gorgon
4. Kill the local king of the place you disembarked
5. Get back home, killing a sea monster along the way.
6. Profit. (Err, no, blessing from Zeus)
Of course, after that, you might have problem with alienating another tribe because one of your questers killed their king, so its best if you "set up" the sea trip to end in an enemy territory already. Identifying and locating the monsters for reenactment purpose could be adventures in itself. PCs could also devise the heroquest from the lore presented to them. Like in "well, we learnt about the myth of Castor and Pollux, the two of us want to reenact that myth...what can we do?" and the end result being appreciated by the closeness of the reenactment... (so something quite removed, but vaguely tied to it would bring a small benefit, while a multistep reenactment could even bring the full-on, six-month of immortality gift [at this power level, resurrection is usually within reach of D&D players, so stablizing them automatically at 0 isn't very powerful.... it would be much more interesting to a runequest character]. Of course if during the end fight of your heroquesting, none of the two questers that took the role of Castor and Pollux die, then the quest is botched, because heroquesting also involved reenacting the mistake. Real myth are complex enough that we have large latitude to imagine re-enactment but I think some in-game myth are developed enough to make it possible if the players are invested in the world's lore. After all, the orcs warrior who pluck his eye out and take his anger against elves, dwarves and humans in turn is just doing a small-scale heroquest to Gruumsh, so there is precedent in 5e's lore ;-)
