Well, some aikidoka have written about kuzushi and a lot of the top guys who came from judo definitely knew about it. What's strange is that they didn't seem to emphasize this concept in training since I've only found one aikido book explicitly mentioning kuzushi (Principles of Aikido by Senta Yamada, who's from the Tomiki line).
I'd also like to point out the fact that the older styles (Iwama, Yoshinkan) work their fundamentals from static positions therefore I'm not sure whether their aikido depends on the energy given by uke.
As for the dojo I train in, we're kind of in between. We sometimes have techniques where the sensei says "ok this one won't work unless you do it in a dynamic situation and take advantage of uke, if he/she doesn't give you what you need you'll have to use *this* technique instead".
Other times, uke resists from a static position and we have to find "the path" using our sensitivity and positioning to push through the "loophole", in the angle where he cannot oppose resistance. That's one of the things you learn from suwariwaza kokyuho.
Here's the basic technique:
Here's an outstanding teacher demonstrating it (skip to 2:20 if you're in a hurry):
We also work on something that I believe is similar to judo kuzushi when we do back breakfalls: uke takes tori's wrist (gyakuhanmi katatedori) and tori extends uke's arm down then behind uke's feet to create an extension and break his balance.
my thought is that in aiki the Tori does break the structure but in a more subtle way than judo. (keep in mind all of these are general statements that do not always apply).
from what i see in Judo there is at times a reversal of force/ energy that facilitates the throw. left to right or front to back. in aiki you dont see that much. you get a more gradual vector that over time/distance the uke loses his structure.
That's an interesting view.
I've also read about a trend in some aikido circles that is geared more towards conceiving aiki as an internal power concept. Their theory is that the religious ramblings of the Founder were actually practical instructions about internal power training that one can decipher by knowing about his cultural and religious background. To them, aiki is the harmony of forces and tensions within the body that in practice makes you able to become immovable, redirect incoming forces and thus create kuzushi on contact with your opponent (just like when you strike at a big bouncing ball and your hand flies away from the impact). Some Daito ryu practicioners, the Founder, Koichi Tohei and Gozo Shioda did similar things.
The problem is that there seems to be no clear methodology for teaching internal skills in aikido and that most of it has been lost by most of the lines, even though there are groups that try to rebuilt it. I have not had hands on training with those people so I cannot tell whether their findings are valuable or not (not that my current level would allow me to have an informed opinion anyway).
One of the most confusing characteristics of aikido is that it varies completely from practicioner to practicioner even within the same line so you end up having a myriad of teachers working on their own aikido, their own concepts, even though in essence we're all doing the same thing. That truly amazes me, you have so many outstanding practicioners from which you can "steal" aikido to build your own. For example, you could look at the way a "fluffy aikido" Hombu teacher pushes into uke's center through his arms and use it to do more "martial" throws: