It sort of disappeared from conversation, but I think the OP's original idea would work quite well with a few tweaks. From what I understand the game client is currently fully capable of 'spawning' completely invisible and untargetable enemies. It does so frequently with things like battlecraft guild leves or maps where enemies spawn at a predetermined event (usually hp% of another mob or some other progress). What actually happens with those things is when you begin a leve, for example, the system loads all relevant mobs, chests, etc. but some may not appear until you trigger them.

What they should do is make invisible, untargetable copies of the marks all over the zone, but rather than have them all spawn together (that might actually be more of an advantage since from nearly anywhere in the zone you'd know that a mark spawned somewhere) make them persistent and allow them to slowly wander the entire zone.

To get around this the radar developers would have to find some way of distinguishing between invisible and visible mobs if that's even possible with the information they can mine. Until they did that the radar app would be completely useless because of the overwhelming number of false positives.