Are you sure? 5.1.1 only patched the bugs we were using, they didn't update the browser. Unless you're saying 5.1.2 is different? (I'm pretty sure they didn't even change the browser in 5.1.2 though)
Based off of the source released by nintendo, it is a build based off of a changelog date of may 15, 2014. If you download it and go into the webkit3.1.1\webkit\webcore folder and view the changelog, the most recent change is:
"2014-05-15 Daniel Bates <[email protected]>"
Unless anyone has more knowledge to add, my guess is this avenue is over until someone finds a webkit hole.