I'd be concerned with balance on it. I really like the idea, however I see it being very important in feedback, since something like a quest victory can easily become too quick to achieve. I mean a master quest would have to be VERY freakishly hard to complete.
I'd also want there to be certain identifiable actions being required in the quest. Like certain artifacts are needed, which are basically forced into several player's hands at the begining of the game, so said players would be forced to infiltrate each other in hopes to steal the artifacts. At which point if one side was loosing the infiltration war, they'd just have to invade as a responce or something like that.
It might be nice to have an "Eye of Sauron" type spell so that you can see how far along those pesky little hobbitses are to quest victory, and even information about how they're going about it (where they're travelling to etc) so that you can set about sabotaging them (at the cost of maintaining the spell obviously).
Oh yes, definattly. But who would get the spell? Would that just be one of those universal spells like "Awareness" in MoM? If so, I'd want an upgraded form of the spell go to some element specifically. (Like nature's awareness was the nature only upgrade to awareness in MoM) Perhaps the spell just shows the progress, but then there is "wispers of the wind" which lets you then also know where they are doing it. I can't think of a better name, but the idea is that everything would be listening for what is happenning. Murders of crows would fly over head and see the heroes, bugs would hear the questers talking about plans, and whatever you can imagine that would create a network of communication back to the wizard casting the spell to let them know of hero progress.
I'm personally a little opposed to a 'spy network' in particular, just because I'm a little burnt out on spys, but the concept of being able to somehow view enemy master quest progression does need to have a form. Most games with spies that come to mind (HoMM, Civ 4 expansions) seem to have rather poor money to info ratios. HoMM requires multiple taverns, but that requires having multiple towns which is pretty hard (sometimes impossible) on some maps. And Civ 4 just makes it a little too subtle for all the money that can be put into it.
I vote on using something like an overland spell that gained more information the longer that it is up. That way you get pretty direct spell vs. info ratio to which we can give easy feedback to SD about. The people doing the spying would be identifiable and countering would be through whatever disjunction means normally available. (something we could probebly start a thread about 
). If a person started nuking the 'eye of sauron' spell (or whatever it is called) about 10 turns in, knowing that in 15 turns it will divulge the wearabouts of some particularly sneaky and important heroes who are all posed to finish the quest. Then players might get tipped off to the fact that this person is doing the quest victory, even though they have not been able to get all the info needed, then alliances might start all throwing up the spell and pumping extra mana into it (something to make it harder to dispell) to try to gain the needed info to properly track the person racing for quest victory.
People going for quest victory as well would want that spell up of their own, just so they can see the progress of others. I could even imagine there would be some trickery of "oh yeah, I don't want this person to get victory. So I have this spell up, and a lot of extra mana on it, so its not gonig to be dispelled. Just trust what I have to say and don't bother putting up spells of your own. (because then you'll see my progress 
) "
