Well, first off, the OP has hit the nail on the head that a hypothetical Ostland was not in the cards given the demography of the Third Reich. Not only were the numbers not really there, but also there was relatively little mass interest within Germany for a recolonization. Although the incipient attempts to colonize both Polish and Soviet territory generated some interest among Germans of an activist bent and speculators, most of the population adapted a wait and see approach to the state's premature announcement to begin colonization of the East in 1941/42. But states can still plan for unrealistic futures and ignore the cold water of difficult facts, and the Third Reich was little different in this regard. The plan, if one could call it that, around the demographic and volunteer shortages to make revolved around three different axes.

Firstly, the great hope for the Generalplan Ost was that the natalist and eugenics policies of the 1930s would reverse demographic decline in German birthrates. This was a longstanding fear in Germany that predated Hitler's seizure of power, and a number of natalist German groups like Reichsbund der Kinderreichen (German Association of Large Families) became quickly co-opted by the Third Reich. The state poured money and resources into propaganda trumpeting the value of large families such as medals for fecund German mothers. But the actual impact of these policies upon the overall birthrate was marginal. The Third Reich did devote some resources to encourage large families, such as rent controls and clothing like HJ-uniforms, but these policies were patchwork and did not deal with the fundamental issues as to why many Germans did not elect for larger families. The state preference for Kinderreich families did not eliminate the financial and personal costs of many children, and the social welfare programs for large families often came at the expense of single and childless German taxpayers. But the wider economic recovery of the mid 1930s did make it appear that these policies had an impact in 1940/41, but in reality, the birthrates were recovering from their Depression lows.

Another hope the Third Reich had to populate this area was the numerous Volksdeustsche colonies and populations who existed outside the borders of the 1939 Reich. Some of the SS's racially charged discourse on the German colonization romanticized Volksdeutsche colonies as rough Germanic types that were not softened by the effete civilization of the West which was unfortunately a necessary evil for Germany's bid for power. The Volksdeutsche were often the colonizers on the ground for the SS, the Volksdeutsche Mittelstelle (Center for Coordination of Ethnic German/VoMi) attempted to arrange for the orderly colonization of the Volksdeutsche into newly conquered areas. VoMi coordinated the Heim ins Reich program which sought to return global Germandom to areas newly conquered by the Reich. As with natalist policies, the Third Reich's Volksdeutsche policies were quite mixed. In order to find the numbers needed, and as a bureaucratic expedient, "Germanic" became much more flexible in practice than it was on paper, and the Volksdeutsche category became quite loose. Moreover, many of VoMi and other state agencies found that Volksdeutsche seldom lived up to their lofty stereotype of honest an Germanic peasant yeomanry. Many Volksdeutsche were disappointed with resettlement when the promised rewards of good buildings and amenities never materialized, and the pervasive corruption within the NSDAP hierarchy meant that much of the plum expropriated territory ended up in the hands of NSDAP cronies or their economic and political allies.

One of the other solutions for the population of an ethnically-cleansed East was not surprisingly technology. Although the Third Reich's propaganda portrayed the future colonizers as a sober Germanic solider-farmer, the hypothetical colonist would enjoy the full benefits of modern conveniences and scientifically-managed farms. The SS's Rasse- und Siedlungshauptamt (Race and Resettlement Bureau/RuSha) spent a good deal of time planning for an ideal, modern farm house and also envisioned that the soldier-farmer would have access to modern agricultural tools. The macro-pattern of RuSha's settlements was that smaller farms would exist in a symbiotic relationship with larger, planned towns, which in turn would be connected to larger administrative garden-cities. This matryoshka-like pattern of settlement would enable the German colonist to be unprecedented in his efficiency as well as connected to all the benefits of modernity without its attending costs.

Naturally, this dreaming of Germany's Eastern vistas proved rather fanciful and some contemporaries noted its unreality at the time. Some Germans coined the term Ostrausch (eastern sickness) to refer to the unhealthy intoxication of remaking the East into a Germanic paradise before the war was even over. Such critics turned out to be correct. The overly ambitious plans meant that rather than building anything, implementing the colonization plans only led to only more death and dislocation.

Sources

Lower, Wendy. Nazi Empire-Building and the Holocaust in Ukraine. Chapel Hill: University of North Carolina Press, 2005.

Mazower, Mark. Hitler's Empire: How the Nazis Ruled Europe. New York: Penguin Press, 2008.

Pine, Lisa. Nazi Family Policy, 1933-1945. Oxford: New York, 1997.