Luther based his translationss on the "Weimarer Kanzleideutsch", which had absorbed considerable influences from High German, even though the varietie spoken in Thuringia and Saxony at the time were Middle German. This is just one example which shows the cultural soft power of the South, where courtly poetry was strong, where greater urbanisation fostered slightly higher literacy in the Late Middle Ages. Just having an alt-Luther flee to Lübeck might not be enough, especially since a Hansa-based support for Protestantism only would have been too little to withstand Habsburg, Wittelsbach etc. presures within the Empire. (It might just as well bring about a Hanseatic oath of loyalty to a Scandinavian king, if he converted, and result in a Danish (or Swedish) northern Germany.)
The only way I see Low German going strong is for the Hansa to remain united earlier, including the towns we now consider Dutch. Northern Germany simply was a thinly populated backwater, but the Netherlands weren't. Historically, Dutch and Low German are parts of a continuum, closer to each other than to High German.
If your alt-Luther goes to an alt-Cologne whose ties to Hamburg and Stralsund are equally close than those to Antwerp and Bruges, and he manages to sway the rhenanian prince-bishoprics for his Protestantism, too, maybe then you might see him standardise German based on a Rhenanian Hanseatic model.