+1 insightful. Also, learning those things is really hard, which is why most ninja don't spend the time. For village ninja it's usually more survivable to learn how to punch harder, stealth better, or blow people up more instead of grinding the ability to transform and then spending all the time to master each individual form. This might or might not apply to you, since you're operating under different constraints.
As an aside, I often feel like we keep all the cool toys away from you guys. The problem is that every time you come up with something super shiny we have to ask ourselves "ninja have been around for centuries. Why did no one think of this?" Whatever answer we come up with has to apply to you as well.
@eaglejarl,
@Velorien: I feel like this is an unreasonable test given the structure of this world.
It's a death world, there's not nearly so many people around to think up and optimize ideas as in our world. Most of the people who are around are mainly concerned with not dying in immediate future, social networks are weak, cities are smaller than what we would consider large towns. Even fewer are ninja, with access to the chakra and jutsu needed to think up the innovations needed. Thinking up clever ideas is hard and often takes experimentation and testing. A mentality sorely lacking the primarily feudal world you've laid out for us, with the possible exception of Orochimaru
1.
But
@HyperCatnip and others have pointed out how good ideas are rare, testing is rare, and bad luck can kill notions in the crib. Which is an important set of things to point out, but I think there's a bigger point that you might be missing.
We don't care about whether someone else has had the idea, just whether the idea existing or not would have a noticeable effect on the world. So to that end I think there's a bit of a checklist that works as a better test.
- How simple is the idea?
- Assuming someone has the prerequisites and wants the goals, how likely are they to think up the idea on their own?
- Something like henge-ing into cloth to slow down would be very simple, but something like building a lattice structure out of 5 seal barriers much less so.
- How available are the prereqs?
- What is the chance that someone will have what the idea needs in order to be tested?
- Something that requires just one of the basic 3 would be highly available (to ninja), but something that requires a high level of sealing and dangerous research would be less available.
- How likely are the goals?
- What is the chance that someone will want the goal the idea works towards?
- Some things, like quickly stopping a long fall are pretty rare since few people are going to be falling long distances in this sort of world, but better single-person defenses or longer distance offensive techniques are incredibly common needs.
If these are all likely then many people have probably thought of the idea. If any of these are unlikely, then few (if any) people have thought of the idea. Then you need to figure out whether the idea has propagated.
- How hard is the idea to test? Does it require many resources?
- How likely is it to propagate? Is it more helpful kept secret or made widely available?
- Are there better solutions available for most people?
- Does the landscape change wildly if only a few people have access to the idea? or only a little?
With the above you can figure out how the notion shifts the large scale landscape. There's also a few other social differences that make your test unreasonable.
This world doesn't have a gentleman's academia like the royal society, much less the professional academia we have now. There's no phone network, and I don't think we've really seen international mail networks. Information flow across large distances seems mostly diplomatic or military. These informational and social structures are incredibly important to the sort of innovation that would merit using a test of "Why hasn't someone else done this yet?" as a first pass.
There's also a strong culture of secrecy. A really good idea might be stuck with a small group of people for fear that releasing it would nix whatever comparative advantage it provides. A lot of good ideas would be squashed by that alone, providing a boost to just a few people rather than a huge change in the status quo. It also makes it harder for someone to build on the work of others.
Not to mention that for sealing in particular, the entire field of engineering is designed to require each individual to do much of the bootstrap work themselves. It's not like fields of engineering where you can literally build on top of the work of others. You need to spend time replicating everything you need, even if you have the work of others to look at. For most people in a village getting to a base level of skill and churning out common seal to sell would be a fast path to relative safety, like most other professions.
You might get the rare privileged seal-crafter with the time, money, and intelligence to do cool research, but it would be hard to pass on to others and kept secret anyway.
The current state of human society is profoundly weird. We've got a level of excess resources, societal trust, and information sharing that is profoundly anomalous. For most of human history technology moved very slowly for a slew of reasons. If MfD is similar, there's no good reason to expect the entire space of low hanging fruit to be searched.
1: I really hope you have an explanation for why Orochimaru felt that striking out on his own was better than accepting both the experimental restrictions and support that would come with good standing as a Konoha nin. There's pros and cons, but that support network seems incredibly useful for most of Orochimaru's canon goals. 
