The method I propose for achieving this is to have each Quest include a list of spells available for use on the HeroQuest or on each Station of the HeroQuest. These are the only spells the HeroQuestor may use whilst on the Quest. Other spells the Questor tries to cast simply will not work. As these spells are naturally part of the Quest, the Questor may cast them more easily:
This actually makes cult magics more powerful on a HeroQuest, which is how things should be.
There is a way that a HeroQuestor may cast spells not normally found on a HeroQuest. The Questor must know a HeroQuest which has the spell as an approved spell. He then Invokes the HeroQuest and casts the spell as normal (using Heroic Castings as necessary). This does not count as a use of the HeroQuest for praying spells back, but as it is a Heroic Casting of the HeroQuest this will cost the Questor MPs equal to the points of the spell. In doing so, the Questor mythically asserts that he has this power, rather than simply using it. Similarly, the use of magics or abilities from magical items is only allowed if the Questor can Invoke a HeroQuest which allows the use of the item.
Spells, abilities or items which have been gained on previous HeroQuests may have the property that they may be used on any related HeroQuest, so a Zorak Zoran cultist may gain a Heroic Shout on one HeroQuest which he may use on Zorak Zoran or Kyger Litor HeroQuests. Such an ability should be usable normally, but would not be able to be used on a Storm Bull Quest without the Originating Quest being Invoked. This is something the GM should decide on a HeroQuest by HeroQuest basis.
As an example, a Zorak Zoran HeroQuestor is on the Hill of Gold Quest and fights Yelmalio. For some reason, he is losing and is about to be impaled by a Fiery Spear. In desperation, he casts a Turn Blow spell, recalling a Xiola Umbar Quest. This is enough to disable his enemy and gives him enough time for victory. Since Xiola Umbar is Zorak Zoran's Sister, this variation of the Quest is not strange enough to attract attention, so he is safe.
This brings us to a question - when is a HeroQuest or HeroPower too different to be acceptable? In my opinion, as long as the Questor has access to a relevant HeroQuest then he may cast any Spell or use any HeroPower without censure. However, certain Quests will fail if particular powers are used - Zorak Zorani may use no Fire Powers on the Hill of Gold Quest, for instance. There is also the problem of using enemy magics - an Orlanthi HeroQuestor may lose kudos if he uses Chaos Powers on an Orlanthi Quest. I would go so far as to say that using overtly enemy powers on a Quest would either lead to the failure of the Quest or to Inactivity or Excommunication when the Quest is over. Of course, Illuminates are probably immune to these effects, but can easily be subject to a sword in the back, after all, overtly using enemy powers in a Quest with other participants is a foolish thing to do.
Hopefully this gives an idea of how I see magic being used in HeroQuests. It ties in well with my other ideas on HeroQuesting and is, in my opinion, a useful addition to the rules set. As I said earlier, anything which restricts the more powerful PCs is a good idea and this fits the bill perfectly. Also, it gives a sound basis for fitting in Magic into HeroQuests.