NASA and Soviet studies on hygiene in preparation for space station missions do not support this conclusion,[28] and no mechanism of action for how the sebaceous glands below the skin detect sebum levels in hair has been proposed.

The given reference #28 seems to be chapter 10 of Packing for Mars... I can't find anything in that book chapter which matches the claim made in the Wikipedia article that no poo and specifically the reduction in sebum has been debunked by both NASA & Russia. The closest passage seems to be

The head in general is a problem. The majority of our sebaceous glands are attached to hair follicles, thus the unwashed scalp quickly becomes a greasy thing. So much so that the bathphobic hordes of the sixteenth century would rub powder or bran into their scalps before retiring for the night, much as homeowners today sprinkle kitty litter on motor oil spills. Like sweat, sebum develops a distinctive aroma as bacteria break it down. “At least two of the Skylab astronauts reported that their heads developed offensive odors,” noted space psychologist Jack Stuster in a 1986 NASA report on space station habitability.

But it also gives a study which suggests no poo could work:

Commander Borman did not wish to discuss skin care. But later, in his memoir, he would write about “our scalps” and about the case of “terminal dandruff” he had. Though it probably wasn’t, technically speaking, dandruff. Dandruff is caused by an inflammatory skin response to oleic acid, which the scalp fungus Malassezia globosa excretes after dining on your scalp oils. Either you’re sensitive to oleic acid or you’re not. If Borman didn’t have dandruff before he went into space, he didn’t have it afterward, says dermatologist Jim Leyden. Leyden once paid prisoners to not wash their hair for a month, specifically to see if they developed dandruff. They did not. The flakes on Borman’s head and skin were most likely the accumulation of millions of shed skin particles—particles normally washed away in the shower—mixing with sebum and clumping together.

And the chapter confirms the 'adjustment' claim for regular hair (although does not specifically claim it applies to the scalp):

Once a set of clothes becomes saturated and oil starts to build up on the skin, what’s the end point? Does uncleansed skin grow ever greasier as the days pass? It does not. According to the Soviet research, the skin halts its production of sebum* after five to seven days of not bathing and not changing one’s increasingly well-greased clothing. Only when the person changes his shirt or takes a shower do the sebaceous glands get back to work. Skin seems happiest with a five-day buildup of oils. Listen to Professor Elaine Larson, editor of the American Journal of Infection Control, talking about the stratum corneum, the outermost layer of human skin: “This horny layer has been compared to a wall of bricks (corneocytes) and mortar (lipids)” and helps “maintain the hydration, pliability, and barrier effectiveness of the skin.”

I have pointed out this discrepancy on the talk page: http://en.wikipedia.org/wiki/Talk:Shampoo#Theory_section:_dubious_use_of_reference