You may not find much because architects are professionals, and aren't trained to write (or think) cogently about social or political issues. 

Inversely, this particular point about the conservatism of regulatory bodies (nearly redundant phrase) is not exclusive to architecture or even the building industry. It's common to professionals. There are fantastic resources out there about the growth of professionalism (versus craft or enterprise) and the material interests of those who represent it. 

Some beginning points: Mary N. Woods, Robert Gutman, Dell Upton, Magali Sarfetti Larson, E.P. Thompson, and I know there are more than a few AIA digs in the Avery Review. Also check out Vitruvius Grind's publication "Welcome to the Grind" for new grads... it may illuminate some things about the structure of the profession that are simply impossible to understand while in school. 

Good luck, and don't be afraid to look somewhere other than the profession for social change ;)

Another kind of response to your query that is perhaps helpful.

Tend to your evidence. Why do you think architecture is (not "can be," throw out all conditional language, it's a manifesto after all) a medium for positive change? What do you point to specifically? What is it that makes the thing you're pointing to "architecture," and what prevents everyone from sharing the same vision?

My suggestion would be that industry groups do not actually share whichever progressive agenda you're assuming. Whose "problem" is it that you're describing? The industry is not monolithic, and problems are not natural phenomena. Lot's of these issues you're looking into are probably functioning exactly as intended. 

Manifestos get a rap for being abstract, but ideology is material at the end of the day.

AIA = regulatory body? …it’s a professional association

Try the State Boards. Maybe get more traction on your search…

NOTE: Speaking from U.S. architectural regulation bodies not that of other countries.

First, AIA in and of itself is not a regulatory body. They do have a degree of influence that can not be denied. This does not mean the AIA has control. The boards of each state are independent of AIA itself. Members of said boards in each state can make decisions independently even against AIA's ideal or that of AIA state and more local chapter units. From the legal perspective, AIA does not control licensing and that was explicitly the point of individual state licensing boards that is under a government agency instead of a private entity. There were concerns that AIA would try to limit those who can be licensed to those who have an architecture degree. At the onset of architectural licensing laws, there were many architects without formal academic degrees. 

While the AIA kind of got its way to a degree over time in a number of states, it wasn't as easy as it would have been if AIA was the licensing authority. This led to states with alternative paths to licensure including experience-based paths to licensure. This was because AIA did not have quite the degree of control as if they had been granted the authority to regulate licensing. This allowed architects who did not go through a 5-year architecture degree such as those who had became an architect through experience in the construction trades and apprenticeship under an architect to gain the proficiency to communicate design and specifications in form of drawings, coordinating projects, etc. eventually setting up shop (office) and offering architectural services in the leading up to architectural licensing laws. Certain grandfathering clauses in the laws allowed those who have been practicing for a period of time (such as 10 years or more) to become licensed without requiring them to meet the normal path to licensing. 

Since the boards are independent of the AIA, this allowed the boards to have some degree of independence and take a different approach than AIA's "plan" (initially and even current plans). Many states had experience-based path to licensure. The number of states that does that has reduced since but still exists. 

While AIA has an influence, they don't outright control. The extent of AIA's influence is the extent of its influence on the individual members serving on the board... which likewise means the individual board members' alignment with AIA's agenda.

We know AIA has its advocacy. It should also be noted that AIA is not the sole voice of the profession. There are other professional associations and advocacy groups of the licensed architectural profession as well as other stakeholders in the broader architectural and related professions that have voices in the various matters affecting the built environment.