You certainly can legislate to promote or inhibit social cohesion. (The family-priority structure of much of the US legal immigration system is one example of how you can promote it, and the poorly-aligned-with-demand per-country/per-category quotas and resulting huge immigration delays in legal, family-based categories are an even better example of how you legislate to inhibit it.) There's plenty of examples (on both sides, because US policy is incoherent as regards this goal) in other policy areas besides immigration.
There are some social residence programs that place minorities/lower-class folks in areas with good schools, high average incomes, etc. If I recall correctly (I heard it on NPR a few months ago) the program had varying degrees of success -- some folks just couldn't fit in and opted to go back previous inner-city residences, while some actually saw measurable improvement in quality of life: children doing better in school, improved outlook and satisfaction.
You probably saw the research results of the crack baby studies -- as it turned out poverty plays an incredibly big role in how children grow up [1]. So I really think this is something we need to continue experimenting with. Sadly, these residency programs cost a lot and most communities are unwilling to fund it. So really the first step is convincing people the reality of these things -- e.g., my neighbor, an incredibly smart biomedical engineer, calls advocates of these programs "troublemakers" who want to "bring socialism" into this country.
