I'm not a huge fan of the ivory bards ideas. When we did the bonestitchers it became apparent that magic is rare enough even asking the living. Besides, if we really wanted a unit that did that I think using apprentices would be a better idea.
I'm also against the idea of using such a system for scouting in the first place though. It seems needlessly complex and expensive for what it does. Someone earlier suggested birds, and I think undead grackles would be a good choice for a scout. Small, quick, observant, common. They would be difficult to detect or stop, but easy to produce.
Special corpses: Corpses of paladin, powerful mages and kings. These corpses are rare, even with those that are being exhumed from the labyrinth beneath the city and those that are smuggled in from outside. Special corpses are used to make truly powerful things. Whether Powerful Units of Death Knights and liches, or Floating monoliths that fire necromantic energy, Flying undead Valkyries or flaming skeletal Giants. Special corpses are the remains of the truly powerful, the hero’s and Tyrants of the world. (Rare, Cannot be gained from recycling)
Magic undead are somewhat confirmed to be an option. An expensive option, yes, but one that seems well-suited to our role as knowledge-seekers. While the examples given are all high-end special-units, they are still made from corpses, so the underlying principals should be consistent, even if we can't pull off anything better than rote-casting a single spell. Yes, it is difficult, but it is also thematic, and could be very powerful if we can manage to pull it off, which will likely require baby-steps. Animating something that requires constant concentration to maintain seems like a very baby-level piece of necromancy to me...
The Ivory Bards offer an intelligent minion that can scout extremely aggressively with little risk and abundant opportunity to report back. Sending something similar to a three-legged dog loping out to claw and bite whatever it can reach while everything it sees is also seen by a much-less-but-still-expendable unit that is likely safely behind a building has utility that is, as far as I can tell, vastly superior to a literally dumb bird. Yes, the bird sees more, but then it either pecks the target to pieces, in which case scouting wasn't needed, gets killed and can't report anything, and lacks enough mental faculty to report anything anyway, or makes a big target of itself by circling over enemies, which is very useful, but still inferior. Now,granted, my proposal looks more difficult, but the experience offered in terms of spell-casting undead seems more thematic to our cause and to have more long-term value than experience gained in the field of animating animals. That said, getting a dead bird to fly is going to be a difficult prospect, and a great chance to learn about incorporating the more difficult aspect of biology into designs.