How about something like in the Rhineland where Poles were shipped in by the tens of thousands?
I don't think that would quite fit the bill; because despite the mass influx of Poles, even the biggest Polish communities rarely reached 10% of the local population. This would mean that they would not be likely to significantly alter the makeup of the area or affect the language enough to 'Slavicize' South Germany. In part, this may be because the infrastructure for such importation of laborers didn't really exist until after the development of nationalism.
On the other hand, if you have a situation where an independent Slavic power, such as an independent Bohemia or South Slavic Austria rules the area of modern-day Bavaria, where they also have converted to Christianity and made nice with the Pope, in the first millenium (perhaps a surviving Great Moravia), then you could see Czechs Moravians, or Austroslavs colonizing Bavaria, which would lead to the OP, though I admit, it wouldn't quite be the same as shipping in Poles from Posen to the Rhineland.