Having only been on the forums for a couple of years before they closed in favor of the G+ community and Facebook page formats, it didn't seem like much discussions happened there in the general forums (there was more happening on the Paizo forums, imo). Towards the end they were getting overrun by spambots more than discussion activity. Prior to the KQ forums, I believed Open Design used Livejournal groups, but that was before I started patroning/backing these projects.
The patron forums were a different matter than the general public forums as those were filled with multi-page brainstorm discussions and critique of patron entries for various design polls. Those were an extensive behind-the-scenes look at the formation of many parts of Midgard (and the planes for Dark Roads and Golden Hells, and the isles and adventures of Journey West and Midgard Tales).
I don't have an archive, but wished I did. There was some spectacular brainstorming in the patron forums. I'm sure the Kobold-in-Chief has an archive, but you know kobolds can be as hoardsome as their dragon progenitors (amongst more real world reasons, since those are patron exclusive forums).
Included in those threads were the designer pitches for various aspects of the setting, be it a region, a deity, an open NPC role. These were the premise for a particular concept the designer established to guide discussions or submissions. The thread would then opened up to brainstorming. Interesting ideas would be incorporated at the designer's discretion in the final draft of the manuscript.
The design surveys would give pitch eligible patrons (narrower selections based on patron tier) a word count with which to compose their taken on a concept. Entries were gathered together and all the patrons would vote for their favorite. Sometimes ties or close calls would create interesting situations.
Going off of memory (so don't hold me to it), many of the secondary leaders or co-rulers were runner-ups in the design polls such as in Triolo, Friula, Valera, and the Despotate (probably others, possibly the Kariv king and queen I think was a tie, the political situation in Melana with the human, kobold and dwarf leadership might have been due to a patron poll result). I also believe the entry for Melana was drafted by a patron using the brainstorm thread, but that was before I joined the project.
Others unfolded a bit differently. The Master of Demon Mountain entries all hewed very close to the same concept, thus the final Master is an amalgam of all the entries (personality, history, characteristics, goals). And it shows with his many names, fittingly enough. The brief summary of some of his children were drawn from a design poll. The Morza and personages of the Dragon Empire was a free for all, somewhere like 20 entries for 8 Dragon Lord spots and a handful of influential NPCs. The Magocracy and the Wasted West emerged a bit differently. Patrons brainstormed and pitched NPCs and conflicts (even the tags on the regional and city map), the designer picked and remixed them as necessary.
The fun part was Wolfgang himself would sometimes pitch along with the patrons and since the entries were presented anonymously, no one could tell if he pitched. He could have easily reserved the space for his own additions (as was sometimes necessary because an idea or position was vital to establishing the tone or premise of an area), but he let some of them compete as normal pitch entries.
Other regions had more written about them (the Crossroads, and the Northlands in particular) and patron input on those were more limited, mostly the merchant and mercenary companies were an open process, others were based on brainstrom. The Domains of the Princes also sprang mostly formed from internal process by the lead designers, the minor noble houses were patron submitted.
Some of the new city maps were sketched by patrons (Valera, Triolo, Reth-Saal, Harkesh, Reywald) and the map tags were a mix of sites off the manuscript and novel additions by the map sketcher (edited by the lead). Other maps existed before (Zobeck and Siwal). Barsella developed from the Journey West project that was underway as the editing of Midgard CS was wrapping up. The planar stuff was inspired by the Dark Roads project that as near complete by the time the MCS was done.
Some of the smaller kingdoms were pitched by high level patrons. A few of the gods were drafted by patrons, including some of the dark gods. There were discussions on each of those to work them into the setting or to add interesting pieces.
On other projects, patrons pitched entire sections and completed them as freelancers for their section (planes for DRGH, islands and adventures for Journeys West, adventures for Tales). While I was not present for earlier projects, I have a feeling this is more the set up for adventure-based projects, with the pitch person actually becoming a freelancer for their adventure.
Currently, the Southlands has the Red Portal forums (using another site). There is a lengthy thread on monster brainstorm, which became for a time a monster workshop as patrons readied their submissions for the Bestiary. The other threads are more in the brainstorm style.