I agree, having a location-dependent MW makes sense. Although as you note it depends on how sensitive the market really is, and everything else equal, rural USA could certainly use a disposable income influx of some kind. Sweden actually has a pretty curious system with no national MW, but strong unions negotiate an implicit per-sector salary floor. This is a bit hit and miss, but has worked fairly well in keeping the working class from lagging too far behind in salary increases. I'm not sure just how flexible it is to regional considerations, but I suspect there is some leeway since it comes down to union-employer negotiations.