Hmmm. Seems like we should find something. According to this site (click) Meg was an ancestor of Sir Walter Scott. Apparently, Robert Browning wrote a poem titled "Muckle-Mouth Meg" - but I couldn't find the text. It should be here (click), but my Internet service is slow this week and I can't load the whole page. Can somebody else try to retrieve and post the poem? Looks like there could be two with this title.
Here's an interesting quote I found at bibliomania.com:

Austrian Lip (The), a protruding under jaw, with a heavy lip disinclined to shut close. It came from kaiser Maximilian I., son of kaiser Frederick III., and was inherited from his grandmother Cimburgis, a Polish princess, duke of Masovia’s daughter, and hence called the “Cimburgis Under Lip.”
A similar peculiarity occurs in the family of sir Gideon Murray of Elibank. He had taken prisoner a young gentleman named Scoto, whom he was about to hang; but his wife persuaded him to commute the sentence into a marriage with their daughter “Meg of the muckle mouth.” Meg made him a most excellent wife, but the “muckle mouth” descended to their posterity for many generations.

Maybe that will help somebody else carry on the search. G'nite.
-Joe Offer-