I have one lighter weight pair of shoes (although not that light) and even those the running store said it would just not be smart to do long stuff in. Simply because they are fine when your form is good, but as you get tired and your form goes down it can be really hard on your body. Not like the ~9x as much pounding (3 miles to 26) isn't hard enough!

Even the running store guy (who is also a coach) doesn't do his long stuff in racers as he just doesn't think it is worth it. I don't think I've seen many people (at least near my speed ;)) that have racing flats in an IM.

I'm curious -- if using the other shoes works for you, why would you want to change it? Is it a matter of that extra bit of weight?

I think a lot of it depends on your body's mechanics and your running form. Whether to use a pair of flats or a full-blown running shoe depends very much on you and your running style. For example, I run all of my mileage on lightweight trainers, even my long runs (20-30k). I am able to do this because 1) I'm 118 pounds, 2) I'm light on my feet, and 3) I'm a pretty efficient runner. So basically, when I go out for a run, my shoes would take less of a pounding than they would if an overpronator who is heavy on their feet were to wear them.

So, you'll probably have to ask yourself about your form and efficiency to see whether or not you would be OK to run a longer race in a flat. Based on the fact that you wear the Kayanos, you're probably a moderate overpronator. Now, are you light on your feet, or is your foot strike a bit heavier? Keep in mind that although this is usually affected by body weight, there are exceptions (I know a few very fit and light people who absolutely pound the ground with every step!) There are several ways to determine this. For example, when you run on the treadmill at an easy run-type pace, are you pretty quiet, or is every step pretty loud? Also, think about how long your shoes usually last you. If you get less than the standard 500 miles out of a shoe, you may be a bit heavier on your feet. If your shoes last a lot longer than this, you are probably pretty efficient.

Based on an assessment like this, you can get an idea of whether or not a flat would work for you in a long race. If you are pretty light on your feet and you are neutral or a normal pronator, you would probably be OK with a racing flat in a marathon. If you are heavier on your feet, or a heavy overpronator, it might be best to stick with your usual trainers. As Kylie pointed out, your form will deteriorate in a marathon, and any imperfections in your form will be intensified.

Ultimately, the way to tell for sure if the shoe will work is to wear them for a long run, preferably well before your goal race. Injuries from improper shoes will surface from repeated running in the shoes, not from one day. You already know that the flats work over a 10k distance, so I doubt that they will do very much damage to you if you take them out once for 30k or so. Even if you find at the end of your test run that the shoes don't have enough cushioning or stability for you at that distance, it is very unlikely that you will be injured as a result of doing one run in the shoes. If the shoes work out over that distance, put them aside until your big race, and enjoy that "fast feel" you have while wearing lighter shoes on race day!

Sorry if this was long and incoherent. I'm a bit of a shoe geek, so I get excited.

I run Long stuff.(halfs and full Mary's ) in the Gel DS Trainer. Still a light shoe but not as light as the Racers...which ,if I'm correct, aren't even listed on their web site anymore.
Mind you...I train in Asics Nimbus, Cumulus, or Nike Vomero because I love cushion...but would never train in the DS Trainer...I think they are too light for that. Race in 'em ...yes. Train? No.
It's always interesting to talk to folks who run long stuff in racing flats...they go fast, but always seem to be injured more.

Best of luck on the qual! You'll have to let us know how it goes :)