I see it being 100% a possibility within our current timeframe, especially with the recent phenomenon of WFH jobs and an increasingly welfare-based economy. An open-system commune could certainly achieve something similar.
It's actually quite simple: Simply repurpose some spiritual text ala Mormonism, and implement the following precepts:
A.) Man and wife should only have sex under two conditions: The first being her at the beginning of her cycle (on her period), the second case being when she is with child. This allows for physical intimacy (see also: oxytocin production at pregnancy term) while preventing/greatly reducing white births.
B.) No adultery should occur between any woman and any other commune member.
C.) As a sacrament, the council of men will raise funds to invite a black bull to come and spend a week or so at the commune. He basically will have the chance to impregnate as many wives as he can in that time. (This goes without saying, of course, but any white commune member has full privilege to watch any white woman be bred during this ceremony.)
D.) Children conceived in this way (halfbreeds) are given every luxury and benefit of the community, but then are released on their 18th birthday. They in turn can act as missionaries to attract further cucks and snow-bunnies to join up.
F.) New members in the commune are invited, but never/rarely born into it (for the above reasons.)
This system would provide an eternal system of a white, productive community, tempered only with monthly bouts of raunchy, interracial sex.