While there are no existing solution for a “native” StumbleUpon plugin for Safari, it is a pretty decent “workaround” to this, that I, myself, find absolutely satisfactory.
To get a “semi-SU-Toolbar” to use while stumbling, like the one in this picture:
Just add a bookmark on your Bookmarks Bar that points to : http://www.stumbleupon.com/su/
After visiting the stubleupon.com website, and setting the “cookie”.
At least that’s something!