Bearded Beast of Duloc
I don't pretend to know all the science, but from my understanding the body does not immediately enter full-blown fat storage mode until it sees a consistent demand.
I will let Casey Butt speak:
Casey Butt: I think all experienced bodybuilders have observed the phenomenon of rapid gains during short periods of overfeeding, particularly when people go on vacation or end a long low-calorie diet. However, the key thing to keep in mind here is the time frame over which the body reacts 'favorably' to the increase in calories.
Very simply, the body exists in essentially two states: energy surplus or energy deficit. In an energy deficit the body will decrease its expression of enzymes responsible for fat storage (lipoprotein lipase, etc), while increasing fat release from adipose cells. This is to provide the body with the energy it needs to maintain metabolism under a calorie deficit (after all, the major purpose of body fat is to provide a source of stored energy). At the same time, in the absence of high insulin levels in response to blood glucose, insulin receptors will uncover on the surface of muscle cells, increasing insulin sensitivity - with little glucose and protein available, the muscle cell will increase it's efficiency of uptake and utilization of these now very much in demand substances. Likewise, growth hormone, IGF-1 and testosterone levels, all of which increase metabolism and lean body mass, decrease when dietary calories are low.
Under these conditions the body has become a very efficient machine at releasing and burning fat from the fat cells and shuttling glucose and nutrients preferentially into the muscle cells (I'm simplifying the situation by neglecting the over-riding needs of the major organs, nervous system, etc).
In a calorie surplus, however, the body will go into a storage mode. In the presence of constantly high insulin, insulin receptors on the surface of muscle cells will "retract" into the cell membrane and become less "sensitive" to the presence of insulin ("insulin resistance"). High levels of fatty acids and glucose in the bloodstream will cause fat cells to up regulate enzymes responsible for fat production and storage. You are now a fat storing machine.
Classic muscle buildingThe key to the preferential deposition of lean body mass, as shown in the above studies during over-eating, lies in the short term nature of the response. Insulin sensitivity and fat storage enzyme activities don't suddenly change dramatically in a few hours or overnight. If the body reacted that quickly to changing environments it would bang back and forth constantly in a never-ending oscillation of metabolically costly adaptations. Evolution isn't that stupid (or perhaps it was but those species never lived that long) and so the adaption to sudden over- or under-eating takes time.
In one of the studies you mentioned (G. Forbes, M. Brown, S. Welle, and L. Underwood, "Hormonal response to overfeeding", Am J Clin Nutr, 1989; 49: 608-11) it was found that elevated testosterone, IGF-1 and insulin levels in response to over-eating all peaked at around 14 days and began to decline thereafter. Perhaps even more importantly, in the presence of high blood sugar and insulin levels fat cells start to increase insulin transporter expression at about two weeks, and that also corresponds with the time it takes for muscle cells to start exhibiting insulin resistance - after a few weeks of over-eating things are starting to go sour.
So, as you can see, the happy situation of preferentially putting on muscle just because you're eating like a pig simply can't last - if it did, formerly obese people would be cleaning up in bodybuilding competitions, but it just doesn't work that way. I over-ate for 18 years and it didn't do anything for me. As I mentioned earlier, bodybuilders often experience quick "rebound" muscle gains when they start eating "normally" again after a long cutting period. In such a circumstance they're coming out of a situation where their muscles are very insulin sensitive and their fat cells are metabolically "geared" to release fat, not store it, and so gains come fast and relatively easily for as long as that situation lasts. A person can experience something similar, though to a lesser degree, by simply over-eating for a few weeks after a period of eating maintenance level calories. However, like I said, you've only got a few weeks to play and then you must pay. If a bodybuilder wants to exploit the anabolic effects of over-eating then I suggest they do it for two weeks or so and then either go on a cut or eat maintenance calorie levels for at least an equal period of time.
As for "dirty bulking" I'd have to say that it's basic macro-nutrients that are most important - calories, carbs, proteins and fats - so "quality" is secondary to "quantity" in this instance. Having said that, it is a known fact that certain dietary components influence testosterone levels and, therefore, could be expected to affect the results of a "bulk". To maximize testosterone levels I'd recommend 30-35% of daily calories from fats with 2/3rds of those being saturated. I'd avoid partially hydrogenated vegetable oils (trans fats) under any circumstances. I'd also recommend at least daily meals of red meat and eggs for the cholesterol, which is crucial to testosterone production in natural athletes. I wouldn't go overboard with protein as excessively high protein intakes decrease testosterone levels. As for carbs, I can't think that it would matter much as to whether "fast" or "slow", "clean" or "dirty" carbs were consumed because with such a high calorie intake, insulin levels would be elevated practically all of the time anyway.
The only significant adjustments I'd make as a long-term eating plan for a natural trainee, other than lowering the calories back down to a sustainable level, would be to limit quick-digesting carbs to breakfast and immediately after training. I'd also try to take in a quick-digesting protein at those times as well. At practically all other times, slow carbs and proteins are the way to go.
"Let bravery be thy choice, but not bravado."