I had commented on this topic once before when it had come up. First let me state that I did go through the training & became certified at administrating colonics. So I'm familiar not on in theory, but in practice.
I've had 4 colonoscopies in my years, all done without any sedation, (no one to drive me home afterwards...). I've experienced a couple types of 'prep' procedures, the laxatives, as well as that gallon of crud to drink. I would still say that the prep is worse than the procedure itself, even without the sedation.
The procedure itself typically only takes 20 minutes, certainly a lot less time compared to sleeping on the toilet the night before.
I watched my last colonoscopy on the large television, and could see that even with the physician's prep orders, that I was not entirely clean. No real stool left in there, but did see a lot of what I can only describe as a green slimy substance.
After previous colonoscopies, I had asked my doctor about, say doing a colonic or enema as a prep. He advised against it, saying that it might tend to inflame in intestinal walls, which may lead to not getting a clear view of what he needs to inspect for.
With my experiences with colonics, sometimes you don't get a full 'clean-out' with those. In theory, a colonic is basically meant to start the natural muscle contractions that would make your system evacuate on it's own. Naturally the water helping to wash the solids out. However there are those times when water will flow up into the intestines, and not all come out as expected.
As with any such procedure, naturally it's best to discuss all options with your doctor. I would certainly opt for a colonic or a series of enemas before the chemical preps prescribed these days, that could be harsh on your system. It seems like a series of enemas, or a colonic would be more natural, however at the cost of these tests as well as the taking time off work, arranging transportation, I'm sure no one would want to have to for got a repeat test because the first was inconclusive.
In these forums, many people has expressed a desire to opt for a colonic or enema as a prep for such tests. However doctors have yet to offer them as an alternative. Doctors will do what is easiest on 'them'. Typically sticking with what has worked in the past, as opposed to trying something different.
I know that when it is time for my next test, I'm certainly going to try harder to convince the doctor to allow me to try some alternative prep, as I will not drink that crud anymore, then spend the night on the toilet, still needing to run to the bathroom before leaving for the test, arriving for the test, changing clothes, even while being wheeled in for the test! From my experience, it just won't stop...
I'm sure people actually look forward to receiving that sedation so they can relax, and not worry about any future 'accidents'. Once on the table and under the sedation, what will be, will be. That's for the doctor to deal with at that point.
I seriously doubt if colonics will even be offered as a prep before such tests. First, they are pretty much unregulated. I believe that there are still only two states that even require a state license to administer colonics. In all the other states, you really don't even need to be certified.
I know in my area, where there are several clinics that offer colonics, they would typically send one person to the school for the certification, then that person would come back and show others how to do it. And it's all totally legal, as again in states that aren't regulated, you don't need the certification to administer a colonic.
I'm sure that doctors can only accept those that have personally been certified, however it's still a very slow process convincing doctors that such things as colonics, or even enemas, are beneficial to ones health.